fix(build): default non-constant untyped shifts

[cpunion]

Summary

• default non-constant untyped basic expressions before go/ssa build so shift-derived parent expressions do not fail sanity checks
• add stable test/go coverage for fixedbugs/issue12133.go and fixedbugs/issue15175.go semantics
• remove the covered goroot xfails for darwin/arm64 and linux/amd64

Tests

• go test ./test/goroot -run TestGoRootRunCases -count=1 -args -goroot /opt/homebrew/Cellar/go@1.24/1.24.11/libexec -dirs fixedbugs -case '^(fixedbugs/issue12133.go|fixedbugs/issue15175.go)$'
• go test ./test/go -count=1
• go test ./internal/build -count=1
• /tmp/llgo-shift test ./test/go

[codecov-commenter]

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Codecov Report

✅ All modified and coverable lines are covered by tests.

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