[X86] Teach LowerBUILD_VECTOR to recognize pair-wise splats of 32-bit elements and...

[X86] Teach LowerBUILD_VECTOR to recognize pair-wise splats of 32-bit elements and use a 64-bit broadcast

If we are splatting pairs of 32-bit elements, we can use a 64-bit broadcast to get the job done.

We could probably could probably do this with other sizes too, for example four 16-bit elements. Or we could broadcast pairs of 16-bit elements using a 32-bit element broadcast. But I've left that as a future improvement.

I've also restricted this to AVX2 only because we can only broadcast loads under AVX.

Differential Revision: https://reviews.llvm.org/D42086

llvm-svn: 322730