fix zero-sized deref comparisons

[cpunion]

Summary

• add nil-deref guards for zero-sized loads and zero-sized field derefs
• keep zero-sized function-result comparisons evaluating calls, including nil function panic behavior
• add focused ssa/cl/test-go coverage and remove fixedbugs/issue23837.go xfail entries

Validation

• go test ./ssa ./cl -run 'TestZeroSized' -count=1
• go run ./cmd/llgo test -run 'TestZeroSize' ./test/go/zero_size_compare_test.go
• go test ./test/go -run 'TestZeroSize' -count=1
• /Users/lijie/sdk/go1.26.0/bin/go test ./test/goroot -run TestGoRootRunCases -count=1 -args -goroot /Users/lijie/sdk/go1.26.0 -go /Users/lijie/sdk/go1.26.0/bin/go -case 'fixedbugs/issue23837.go' -xfail /tmp/llgo-empty-xfail.yaml
• also verified the same GOROOT case with Go 1.24.11 and Go 1.25.0 locally

Note: package-wide llgo test -run 'TestZeroSize' ./test/go currently fails at link time on existing generic_local_types symbols; the added zero-size test file passes directly.

[codecov-commenter]

⚠️ Please install the to ensure uploads and comments are reliably processed by Codecov.

Codecov Report

✅ All modified and coverable lines are covered by tests.

📢 Thoughts on this report? Let us know!