fix nil receiver method dispatch checks

[cpunion]

Summary

• Emit explicit nil-deref guards for Go pointer indirections, while preserving the array-pointer range skip.
• Implement ssa:wrapnilchk so value-receiver method wrappers panic on nil pointer receivers.
• Add focused test/go coverage for promoted methods, method expressions/values, and interface calls on nil receivers.
• Remove the fixed method.go and method5.go GOROOT xfails.

Testing

• go test ./cl -run 'TestCompileSmallNilDerefGuards|TestCompileLargeNilDerefInterfaceGuards|TestIsLargeNonPointerValue' -count=1
• go test ./test/go -run 'Test(Promoted|ValueReceiver|InterfaceCall)' -count=1
• ./dev/llgo.sh test -run 'Test(Promoted|ValueReceiver|InterfaceCall)' ./test/go
• go test ./test/goroot -count=1 -timeout 20m -run TestGoRoot -args -goroot "$(go env GOROOT)" -dirs . -case '^method\.go$' -xfail /tmp/llgo-no-xfail.yaml
• go test ./test/goroot -count=1 -timeout 20m -run TestGoRoot -args -goroot "$(go env GOROOT)" -dirs . -case '^method5\.go$' -xfail /tmp/llgo-no-xfail.yaml
• go test ./test/goroot -count=1 -timeout 20m -run TestGoRoot -args -goroot "$(go env GOROOT)" -dirs . -case '^method(5)?\.go$'

GOROOT CI is slow/disabled, so this PR keeps stable coverage in test/go and lists the exact targeted GOROOT commands above. I did not push any branch to xgo-dev/llgo; normal PR CI should run from the fork branch.

[cpunion]

Updated this draft PR with commit 90947ed92164517dff3dab36c3ea98b3e5cdd942 pushed to cpunion/llgo:codex/goroot-method-coverage.

Root cause: the initial nil receiver method-dispatch change guarded ordinary pointer dereferences too broadly, which shifted existing FileCheck output, while the real missing behavior was value-receiver dispatch through nil pointer/promoted embedded pointer paths that must panic under Go semantics.

Fix: narrow the nil-deref guard to the method dispatch cases that need Go-compatible panic behavior and update the affected FileCheck expectations.

Local verification reported from the worktree:

• go test ./cl ./ssa ./test/go -count=1

CI has been re-triggered on the new head; I will continue monitoring it.

[cpunion]

Update pushed to cpunion:codex/goroot-method-coverage at 251419cb0360b1dc3cf16839669e821a91359549.

Root cause: the failed CI job was testing the GitHub PR merge commit, not just the branch head. Current xgo-dev/main has a newer cl/_testgo/genericembediface FileCheck test that was not present in the branch head. After this PR's nil method dispatch changes, the generated value-receiver wrappers now include runtime.AssertNilDeref, so the base test's old IR expectations failed in the merge commit.

Fix:

• merged current xgo-dev/main into the PR branch so the base-only test is present on the branch without add/add merge risk;
• updated cl/_testgo/genericembediface/in.go expectations for (*server).ServerReflectionInfo and (*stream).Context to include the nil deref checks;
• kept behavior coverage in test/go/method_dispatch_test.go from the previous commit.

Local verification:

• go test ./ssa -run 'TestFromTestgo/genericembediface' -count=1
• go test ./cl -run 'TestRunAndTestFromTestgo/genericembediface' -count=1
• go test ./test/go -count=1
• go test ./cl ./ssa -count=1

CI status: new head checks are running. The replacement for the failed job is Go / test (ubuntu-latest, 19) at https://github.com/xgo-dev/llgo/actions/runs/26305192373/job/77440063804 and is currently in progress/queued. PR is still draft.

[cpunion]

CI update for head 251419cb0360b1dc3cf16839669e821a91359549:

• The replacement for the previous failure, Go / test (ubuntu-latest, 19), completed successfully: https://github.com/xgo-dev/llgo/actions/runs/26305192373/job/77440063804
• Its Test step (go test -timeout 30m ./...) passed, so the merge-only FileCheck failure is resolved.
• Current checks summary shows no failures; remaining macOS jobs are still pending/queued.