ssa: handle recursive pointer type names

[cpunion]

Summary

• avoid recursively resolving the LLVM element type for Go pointer types, which fixes self-referential named pointer types such as type Number *Number
• add test/go coverage that runs a recursive named pointer type through llgo, including a 4096-deep Peano construction/count matching fixedbugs/issue4316.go
• remove the now-covered peano.go and fixedbugs/issue4316.go goroot xfails for darwin/arm64 and linux/amd64

Scope

• Covered: recursive named pointer / Peano / recursion lowering cases.
• Not covered here: nil/chan/print/recover/GC/liveness/finalizer/goroutine domains; those remain out of scope for this PR.

Tests

• go test ./test/goroot -count=1 -run TestGoRootRunCases -timeout 10m -args -goroot "$(go env GOROOT)" -dirs fixedbugs -case '^fixedbugs/issue4316\.go$' -xfail /dev/null -run-timeout 2m
• go test ./test/goroot -count=1 -run TestGoRootRunCases -timeout 10m -args -goroot "$(go env GOROOT)" -dirs . -case '^peano\.go$' -xfail /dev/null -run-timeout 2m
• go test ./test/goroot -count=1 -run TestGoRootRunCases -timeout 15m -args -goroot "$(go env GOROOT)" -dirs .,fixedbugs -case '^(peano\.go|fixedbugs/issue4316\.go)$' -xfail /dev/null -run-timeout 2m
• go test ./test/go -run TestRecursivePointerTypeBuilds -count=1
• go test ./test/go -count=1
• go test ./ssa -count=1
• go test ./cl -list .
• go test ./cl -run TestRunAndTestFromTestgo -count=1 -timeout 5m

Local test notes

• go test ./cl -count=1 was also attempted locally and was terminated after 428s without a Peano/named-pointer failure.
• go test ./cl -run 'TestRunAndTestFromTestgo|TestRunAndTestFromTestrt' -count=1 -timeout 5m timed out in TestRunAndTestFromTestrt/index; this is outside the recursive named pointer / Peano scope.

[cpunion]

Update after expanding Peano coverage:

• Classified fixedbugs/issue4316.go as the same recursive named pointer / Peano family as peano.go; it passes on the current PR head with xfail bypassed.
• Added test/go coverage for 4096-deep type Peano *Peano construction/counting.
• Removed fixedbugs/issue4316.go xfails for darwin/arm64 and linux/amd64.
• Left nil/chan/print/recover/GC/liveness/finalizer/goroutine domains untouched.

Local validation passed for focused goroot Peano + issue4316, go test ./test/go -count=1, go test ./ssa -count=1, and go test ./cl -run TestRunAndTestFromTestgo -count=1 -timeout 5m. Full cl/testrt long-run attempts hit local termination/timeout in testrt/index, outside this PR scope.

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