[X86] Demote EmitTest to a helper function of EmitCmp. Route all callers...

[X86] Demote EmitTest to a helper function of EmitCmp. Route all callers except EmitCmp through EmitCmp.

This requires the two callers to manifest a 0 to make EmitCmp call EmitTest.

I'm looking into changing how we combine TEST and flag setting instructions to not be part of lowering. And instead be part of DAG combine or isel. Which will mean EmitTest will probably become gutted and maybe disappear entirely.

llvm-svn: 349094