cmd/compile: expand LLVM nil checks after IR construction

[zhouguangyuan0718]

Summary

• emit llvm.goallc.nilcheck as a temporary semantic marker during SSA-to-LLVM value lowering
• finish the original LLVM CFG and populate PHI incoming edges through the normal FinishPhi() path
• expand every marker afterward into an explicit null branch and ABIInternal runtime.panicmem call
• repair only the affected successor PHI predecessors through Value.ReplaceIncomingBlock
• remove the first-pass BlockEnds CFG bypass entirely
• fail closed if the marker declaration/type/calls are malformed or if any marker survives expansion

The marker is deleted before LLVM verification, optimization, statepoint rewriting, or IR emission.

Uses merged goallc/go-llvm#5 at v0.0.0-20260802014221-e0a85f0a18bc (master@e0a85f0a18bc).

Validation

• src/make.bash
• go mod verify; go list -mod=vendor github.com/goallc/go-llvm
• go test -tags="llvm23 dynamicllvm" -count=1 cmd/compile/internal/ssa cmd/llvmtoolexec
• LLVM codegen whitelist 49/49
• LLVM runtime whitelist 46/46
• focused nilcheck codegen, runtime/recover, and GoObj tests
• goallc/go-llvm full tests
• plugin CTest 47/47
• final IR and statepoint IR contain no llvm.goallc.nilcheck
• opt -load-pass-plugin=... -passes=verify
• llc -load-pass-plugin=... -verify-machineinstrs -filetype=obj
• objview confirms frameful ordinary panicmem safepoint map and separate stack-growth map
• gofmt; go mod tidy; go mod vendor; git diff --check

Existing boundary

The aggregate Test/LLVM command still reports the pre-existing abi-differential/mixedABI pointer-map count mismatch. The nilcheck fixture and changed lowering do not participate in that ABI case; codegen, runtime, alloca/statepoint, and fail-closed groups otherwise pass.