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 62.16216% with 14 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
cl/compile.go	62.16%	13 Missing and 1 partial ⚠️

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