Patch to fix vselect compilation on KNL.

[Bader, Aleksey A]

Hi LLVM developers,

I need your help with reviewing and committing the patch.

Build of that program:
define <16 x i32> @test() {
entry:
  %0 = icmp slt <16 x i32> undef, undef
  %1 = select <16 x i1> %0, <16 x i32> undef, <16 x i32> zeroinitializer
  ret <16 x i32> %1
}
Fails on KNL because of optimization in PerformSELECTCombine, which replaces (x < y) ? a : 0 => (x < y) & a.
It seems to be profitable only if we keep comparison result in the same register group as 'a' (i.e. for vector architectures without dedicated mask registers).
There is no need in this optimization on KNL because vselect can be implemented as masked move (single instruction).

Thanks in advance,
Alexey

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