[InstCombine] canonicalize a scalar-select-of-vectors to vector select

This pattern may arise more frequently with an enhancement to SLP vectorization suggested in PR42755:
https://bugs.llvm.org/show_bug.cgi?id=42755
...but we should handle this pattern to make things easier for the backend either way.

For all in-tree targets that I looked at, codegen for typical vector sizes looks better when we change
to a vector select, so this is safe to do without a cost model (in other words, as a target-independent
canonicalization).

For example, if the condition of the select is a scalar, we end up with something like this on x86:

	vpcmpgtd	%xmm0, %xmm1, %xmm0
	vpextrb	$12, %xmm0, %eax
	testb	$1, %al
	jne	LBB0_2
  ## %bb.1:
	vmovaps	%xmm3, %xmm2
  LBB0_2:
	vmovaps	%xmm2, %xmm0

Rather than the splat-condition variant:

	vpcmpgtd	%xmm0, %xmm1, %xmm0
	vpshufd	$255, %xmm0, %xmm0      ## xmm0 = xmm0[3,3,3,3]
	vblendvps	%xmm0, %xmm2, %xmm3, %xmm0

Differential Revision: https://reviews.llvm.org/D66095

llvm-svn: 369140