Fix a problem with DAGCombine on 64b targets where folding

extracts + build_vector into a shuffle would fail, because the
type of the new build_vector would not be legal.  Try harder to
create a legal build_vector type.  Note: this will be totally 
irrelevant once vector_shuffle no longer takes a build_vector for
shuffle mask.

New:
_foo:
	xorps	%xmm0, %xmm0
	xorps	%xmm1, %xmm1
	subps	%xmm1, %xmm1
	mulps	%xmm0, %xmm1
	addps	%xmm0, %xmm1
	movaps	%xmm1, 0

Old:
_foo:
	xorps	%xmm0, %xmm0
	movss	%xmm0, %xmm1
	xorps	%xmm2, %xmm2
	unpcklps	%xmm1, %xmm2
	pshufd	$80, %xmm1, %xmm1
	unpcklps	%xmm1, %xmm2
	pslldq	$16, %xmm2
	pshufd	$57, %xmm2, %xmm1
	subps	%xmm0, %xmm1
	mulps	%xmm0, %xmm1
	addps	%xmm0, %xmm1
	movaps	%xmm1, 0

llvm-svn: 65791