[DAGCombiner][X86][Mips] Enable combineShuffleOfScalars to run between vector...

[DAGCombiner][X86][Mips] Enable combineShuffleOfScalars to run between vector op legalization and DAG legalization. Fix bad one use check in combineShuffleOfScalars

It's possible for vector op legalization to generate a shuffle. If that happens we should give a chance for DAG combine to combine that with a build_vector input.

I also fixed a bug in combineShuffleOfScalars that was considering the number of uses on a undef input to a shuffle. We don't care how many times undef is used.

Differential Revision: https://reviews.llvm.org/D54283

llvm-svn: 346530