Fix slice-to-array conversion panics

[cpunion]

Summary

• pass slice-to-array conversion panic helper arguments as target length, slice length
• preserve nil-deref side effects for unused slice derefs produced by zero-length slice-to-array conversions
• make explicit nil-deref assertions recover as runtime errors, matching signal-based nil pointer panics
• add focused test/go coverage and remove fixed convert4.go goroot xfails

Tests

• go test ./test/go -run 'TestSliceToArrayConversion' -count=1
• go test ./ssa -count=1
• go test ./cl -run 'TestRunAndTestFromTestrt|TestSkipUnusedArrayDeref' -count=1
• (cd runtime && go test ./internal/runtime -count=1)
• go run ./cmd/llgo test -run 'TestSliceToArrayConversion' ./test/go
• go test ./test/goroot -count=1 -run TestGoRootRunCases -args -goroot "$(go env GOROOT)" -dirs . -case '^convert4\.go$' -directive-mode legacy -xfail /tmp/llgo-empty-xfail.yaml -progress 30s
• go test ./test/goroot -count=1 -run TestGoRootRunCases -args -goroot "$(go env GOROOT)" -dirs . -case '^convert4\.go$' -directive-mode legacy -progress 30s

Notes

• Local GOROOT source: $(go env GOROOT) ($(go env GOVERSION), $(go env GOOS)/$(go env GOARCH)).
• Full GOROOT CI is slow/disabled; this PR relies on normal PR CI plus the targeted convert4.go goroot run above.
• Stable coverage is in test/go/slice_array_convert_test.go, so the fix is not covered only by test/goroot.

[codecov-commenter]

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Codecov Report

❌ Patch coverage is 98.21429% with 1 line in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
cl/compile.go	87.50%	0 Missing and 1 partial ⚠️

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[cpunion]

Conversion-only update for this draft PR.

Changes in f1f0a989d18194c1f220fd83cf858db7aba17149:

• Fixed float-to-integer conversion bounds in SSA codegen: signed targets saturate with NaN -> 0; uint32/uint64/uintptr clamp negative/NaN to 0 and overflow/Inf to max; narrow unsigned targets keep the signed-intermediate truncation behavior needed for Go compatibility.
• Added stable test/go coverage in TestFloatToIntegerConversionSemantics.
• Updated IR/runtime fixtures affected by the conversion lowering.
• Removed goroot xfails for convert5.go and convinline.go. convert4.go was already passing. Kept only the existing darwin convinline.go timeout entries because those are timeout controls, not conversion failures.

Goroot case status:

• convert4.go: passes without xfail.
• convert5.go: passes without xfail after this fix.
• convinline.go: passes without xfail; this shared the same float-to-int conversion root cause, not a separate runoutput generation fix.

Local validation:

• go test ./test/go -count=1
• go test ./ssa -count=1
• go test ./cl -count=1
• go test ./ssa -run TestFromTestrt/cast -count=1 -v
• go test ./cl -run TestRunAndTestFromTestrt/cast -count=1 -v
• go test ./test/goroot -run TestGoRootRunCases -count=1 -args -goroot /Users/lijie/sdk/go1.26.0 -dirs . -case '^(convert4|convert5|convinline)\\.go$' -directive-mode ci -xfail /tmp/llgo-no-xfail.yaml -run-timeout 4m -progress 30s
• Previously also spot-checked convinline.go without xfail on local Go 1.24.11 and Go 1.25.0.

PR is still draft. CI has started on the new head.