[X86] Add test cases for suboptimal vselect+setcc splitting.

[X86] Add test cases for suboptimal vselect+setcc splitting.

If the vselect result type needs to be split, it will try to
also try to split the condition if it happens to be a setcc.

With avx512 where k-registers are legal, its probably better
to just use a kshift to split the mask register.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@373536 91177308-0d34-0410-b5e6-96231b3b80d8