[X86] In lowerVectorShuffle, instead of creating a new node to canonicalize...

[X86] In lowerVectorShuffle, instead of creating a new node to canonicalize the shuffle mask by commuting, just commute the mask and swap V1/V2.

LegalizeDAG tries to legal the DAG by legalizing nodes before
their operands.

If we create a new node, we end up legalizing it after its operands.
This prevents some of the optimizations that can be done when the
operand is a build_vector since the build_vector will have been
legalized to something else.

Differential Revision: https://reviews.llvm.org/D65132

llvm-svn: 366835