goroot: reduce xfail backlog across go1.24-go1.26

[cpunion]

Summary

This PR continues the GOROOT runner and xfail cleanup on top of #1759.

Current scope stays limited to:

• Go 1.24.11, 1.25.0, and 1.26.0
• linux/amd64, linux/arm64, and darwin/arm64 accounting
• ci discovery mode for the active runner surface

Included

• runtime/SSA fixes and xfail cleanup carried from the earlier commits in this branch
• widened GOROOT run-case discovery for CI-oriented run directives
• xfail, host-skip, timeout, and flaky metadata for the expanded runner surface
• Go 1.26 new(expr) support via golang.org/x/tools/go/ssa v0.38.0, which emits the required store after the allocation
• removal of the now-stale newexpr.go xfails for go1.26 darwin/arm64 and go1.26 linux/amd64
• Go 1.24 linux/amd64 flaky metadata for fixedbugs/issue52612.go, matching existing Go 1.25/1.26 behavior
• additional SSA unit coverage for type-equivalence and inline/index edge cases to recover project coverage

Current Inventory

Current test/goroot/xfail.yaml inventory in this branch:

• 477 xfails
• 12 flaky entries
• 18 host skips
• 269 timeout overrides
• 776 total metadata entries

Current ci mode runner accounting after host-skip, xfail, and flaky matching:

Platform	Go version	Total cases	Host skips	Xfails	Flaky
linux/amd64	go1.24.11	1027	2	102	2
linux/amd64	go1.25.0	1035	3	104	3
linux/amd64	go1.26.0	1047	2	119	4
darwin/arm64	go1.24.11	1023	7	103	1
darwin/arm64	go1.25.0	1031	7	105	1
darwin/arm64	go1.26.0	1043	7	120	1
linux/arm64	go1.24.11	1025	0	0	0
linux/arm64	go1.25.0	1033	0	0	0
linux/arm64	go1.26.0	1045	0	0	0

For go1.26, remaining xfail functional categories are:

Platform	builtin/layout	channels/select	debug/trace	generics	language semantics	map/interface equality	methods/reflection	panic/defer	runtime/gc/profiler	other
linux/amd64	1	2	6	9	3	1	5	7	4	81
darwin/arm64	1	4	7	9	2	1	5	8	4	79

For go1.26, host-skip and flaky functional categories are:

Kind	Platform	Categories
Host skip	linux/amd64	channels/select 1, other 1
Host skip	darwin/arm64	channels/select 2, runtime/gc/profiler 1, other 4
Flaky	linux/amd64	debug/trace 1, other 3
Flaky	darwin/arm64	other 1

Runner Modes

test/goroot keeps three discovery modes:

• legacy: original narrow mode, only bare // run
• ci: current CI mode, covering run, run <args>, runoutput, rundir, runindir, and buildrun
• runlike: broader local mode that also includes buildrundir

The active CI validation continues to use ci mode. runlike remains local/experimental until the known buildrundir blockers are fixed.

Known buildrundir blockers from the broader local expansion:

• retjmp.go: translated asm return-jump semantics still diverge with panic: f2 not called
• asmhdr.go: plan9asm constant expansion still rejects string consts with panic: "test" not an Int

Validation

Latest local validation:

• git diff --check
• go test ./test/goroot -count=1 -run 'Test(FlakyMatch|XFailMatch|HostSkipMatch|TimeoutMatch)'
• docker run --rm --platform linux/amd64 -v "$PWD":/workspace -v llgo-empty-crosscompile:/workspace/crosscompile -w /workspace llgo-wt-goroot-xfail-priority-llgo-dev-amd64:latest bash -lc 'go test ./test/goroot -count=1 -timeout 30m -run TestGoRootRunCases -args -goroot /workspace/.cache/goroot-linux-sdks/go1.26.0 -directive-mode ci -xfail /tmp/no-such-xfail.yaml -case "^newexpr\\.go$"'
• docker run --rm --platform linux/amd64 -v "$PWD":/workspace -v llgo-empty-crosscompile:/workspace/crosscompile -w /workspace llgo-wt-goroot-xfail-priority-llgo-dev-amd64:latest bash -lc 'go test ./test/goroot -count=1 -timeout 30m -run TestGoRootRunCases -args -goroot /workspace/.cache/goroot-linux-sdks/go1.26.0 -directive-mode ci -case "^newexpr\\.go$"'
• /Users/lijie/sdk/go1.26.0/bin/go test ./test/goroot -count=1 -timeout 30m -run TestGoRootRunCases -args -goroot /Users/lijie/sdk/go1.26.0 -go /Users/lijie/sdk/go1.26.0/bin/go -directive-mode ci -case '^newexpr\\.go$'
• docker run --rm --platform linux/amd64 -v "$PWD":/workspace -v llgo-empty-crosscompile:/workspace/crosscompile -w /workspace llgo-wt-goroot-xfail-priority-llgo-dev-amd64:latest bash -lc 'go test ./test/goroot -count=1 -timeout 90m -run TestGoRootRunCases -args -goroot /workspace/.cache/goroot-linux-sdks/go1.24.11 -directive-mode ci -shard-index 3 -shard-total 4'
• go test ./ssa -count=1 -run 'Test(FunctionInlineAttributes|IndexRejectsInvalidTypes|NamedTypeEquivalent|NamedStructLayoutEquivalent|FuncName_AliasPointerReceiverUsesUnderlyingNamedType)'
• go test ./ssa -coverprofile=/tmp/llgo-ssa-after3.cover -covermode=atomic (94.9%, +13 covered statements vs the prior local profile)
• GOTOOLCHAIN=local go install ./cmd/llgo
• PATH="$(go env GOPATH)/bin:$PATH" GOTOOLCHAIN=local bash -lc 'source doc/_readme/scripts/run.sh' exited 0; the script still prints existing local Python 3.14 SRE module mismatch/segfault diagnostics because it does not run with set -e, matching CI semantics

GitHub validation for head b259f3c7cd2a32c76f470d87358dba234cfca380:

• PR checks: 81 passed, 1 skipped (release), 0 pending, 0 failed
• codecov/project and codecov/patch passed

Broader local package validation from this iteration:

• go test ./internal/build ./cl ./ssa -count=1 passed internal/build and ssa; local cl timed out after 10 minutes in TestRunFromTestlibc/setjmp, which is outside the newexpr.go path and should be treated as a host/local timeout rather than a passing validation.

Known follow-up categories to prioritize next:

• remaining builtin/layout: cmplxdivide.go
• language semantics: nilptr.go, range4.go, rangegen.go
• layout/runtime edge cases currently grouped under other