[X86] Fix two more places to prefer VPERMQ/PD over VPERM2X128 when AVX2 is enabled

The shuffle combining and lowerVectorShuffleAsLanePermuteAndBlend were both still trying to use VPERM2XF128 for unary shuffles when AVX2 is enabled. VPERM2X128 takes two inputs meaning when we use it for a unary shuffle one of those inputs is left undefined creating a false dependency on whatever register gets allocated there.

If we have VPERMQ/PD we should prefer those since they only have a single input.

Differential Revision: https://reviews.llvm.org/D37947

llvm-svn: 313542