[TargetLowering] Rename DAGCombinerInfo::isAfterLegalizeVectorOps to...

[TargetLowering] Rename DAGCombinerInfo::isAfterLegalizeVectorOps to DAGCombiner::isAfterLegalizeDAG since that's what it checks. NFC

The code checks Level == AfterLegalizeDAG which is the fourth and last of the possible DAG combine stages that we have.

There is a Level called AfterLegalVectorOps, but that's the third DAG combine and it doesn't always run.

A function called isAfterLegalVectorOps should imply it returns true in either of the DAG combines that runs after the legalize vector ops stage, but that's not what this function does.

llvm-svn: 326832