[X86] Lower vselect with constant condition to vector_shuffle even with AVX512 instructions.
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
Differential Revision: https://reviews.llvm.org/D38932