cmd/llvmplugin: use standard annotations for nil checks

[zhouguangyuan0718]

Summary

• encode frontend nil checks with standard LLVM annotation metadata
• recognize that annotation in the statepoint plugin after optimization
• remove the earlier custom nilcheck marker contract
• add O2 and pointer-root regression coverage

Dependency

Requires goallc/llvm-project#20 so SROA preserves load annotations while rewriting aggregates. It is otherwise independent of write-barrier lowering in Go PR #32.

Validation

• focused compiler IR check for annotation metadata on the volatile nilcheck load
• GoALLCStatepoints AllocaPointerRoots
• GoALLCStatepoints AllocaNilCheckO2 with LLVM cmd/internal/testdir: cover LLVM amd64 argument maps #20
• integrated 50/50 LLVM codegen matrix
• gofmt and git diff --check