[LegalizeTypes][X86] When splitting a vselect for type legalization, don't...

[LegalizeTypes][X86] When splitting a vselect for type legalization, don't split a setcc condition if the setcc input is legal and vXi1 conditions are supported

Summary: The VSELECT splitting code tries to split a setcc input as well. But on avx512 where mask registers are well supported it should be better to just split the mask and use a single compare.

Reviewers: RKSimon, spatel, efriedma

Reviewed By: spatel

Subscribers: hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D68359

llvm-svn: 373863