fix nil switch lowering

[cpunion]

Summary

• lower untyped nil SSA constants with comparison/conversion context so switch case nil codegen no longer tries to materialize untyped nil directly
• add stable test/go coverage for nil switch cases across funcs, maps, slices, channels, and typed nil pointers
• remove the fixed darwin/arm64 switch.go GOROOT xfail entry

Notes

GOROOT CI is slow/disabled for broad runs, so this PR adds focused test/go coverage for the fixed behavior and only runs the targeted GOROOT case locally.

Targeted validation

• go test ./test/go -run TestSwitchNilCases -count=1
• go test ./cl -run 'Test.*Nil|Test.*Builtin|Test.*Cgo|Test.*Compile' -count=1
• go run -tags=dev ./cmd/llgo test -a -run TestSwitchNilCases ./test/go
• LLGO_BUILD_CACHE=off go test ./test/goroot -run TestGoRootRunCases/switch.go -count=1 -timeout 10m -args -goroot "$(go env GOROOT)" -dirs . -case '^switch\.go$' -xfail /tmp/llgo-empty-xfail.yaml -run-timeout 3m -build-timeout 5m -progress 15s
• LLGO_BUILD_CACHE=off go test ./test/goroot -run TestGoRootRunCases/switch.go -count=1 -timeout 10m -args -goroot "$(go env GOROOT)" -dirs . -case '^switch\.go$' -run-timeout 3m -build-timeout 5m -progress 15s

[codecov-commenter]

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Codecov Report

❌ Patch coverage is 30.30303% with 23 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
cl/compile.go	30.30%	18 Missing and 5 partials ⚠️

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[cpunion]

Update for commit 706507a:

• Classified fixedbugs/issue53635.go as covered by the fix nil switch lowering #1883 nil switch/compare lowering path: the current branch passes the case without xfail, and the stale xfail now reports unexpected success.
• Added stable test/go coverage for generic []T, func() T, and map[int]T nil switch and direct nil comparison cases.
• Removed only the fixed fixedbugs/issue53635.go xfail entries for darwin/arm64 and linux/amd64.
• Did not cover or touch chan, nil panic, print, recover, GC/liveness/finalizer/goroutine work, or runtime: run BDWGC finalizers after collection #1881/fix reflect MakeFunc goroutine liveness #1900/runtime: add minimal MemProfile allocation records #1902/runtime: add memprofile attribution support #1905/Fix stack object finalizer liveness #1906. No fix generic local runtime type names #1879 split needed because this reproduces as the same untyped-nil lowering class rather than a typeparam-only root.

Local validation:

• go test ./test/goroot -run TestGoRootRunCases -count=1 -args -goroot /Users/lijie/sdk/go1.26.0 -dirs fixedbugs -case '^fixedbugs/issue53635.go$' -run-timeout 60s
• go test ./test/go -count=1
• go test ./cl ./ssa -count=1
• go run ./cmd/llgo test -run TestGenericSwitchNilCases ./test/go -count=1

Runtime package tests were not run because no runtime files or runtime behavior were changed.