cmd/compile: preserve KeepAlive through LLVM inlining

[zhouguangyuan0718]

Lower OpKeepAlive and pointer-bearing OpVarLive to an llvm.donothing call carrying a go.keepalive operand bundle. The bundle is a real SSA use that survives LLVM inlining and O2, while llvm.donothing emits no machine instruction. This avoids changing llvm.fake.use or generic LLVM inliner behavior.

The statepoint liveness scan sees operand-bundle inputs, so an inlined KeepAlive keeps the referenced stack object in pointer maps until the source keepalive point.

Depends on goallc/go-llvm#8. The matching go-llvm binding is vendored here.

Tests:

• go test -tags=llvm23,dynamicllvm cmd/compile/internal/ssa -count=1
• go test cmd/internal/testdir -run Test/LLVM/codegen/codegen/llvm_memory_order.go -count=1
• go test cmd/internal/testdir -run Test/LLVM/runtime/stackobj(2)?.go -count=1
• go test cmd/internal/testdir -run ^Test$/^LLVM$/^codegen -count=1