[X86] Lower vselect with constant condition to vector_shuffle even with AVX512 instructions.

Summary:
It's better to use our shuffle lowering code to handle these than loading an immediate into a k-register.

It really feels like this should be a DAG combine optimization rather than a lowering operation, but that's a problem for another day.

Reviewers: RKSimon, delena, zvi

Reviewed By: delena

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D38932

llvm-svn: 315849