teach libanalysis to simplify vector loads with bitcast sources. This

teach libanalysis to simplify vector loads with bitcast sources.  This
implements something out of Target/README.txt producing:

_foo:                                                       ## @foo
	movl	4(%esp), %eax
	movapd	LCPI1_0, %xmm0
	movapd	%xmm0, (%eax)
	ret	$4

instead of:

_foo:                                                       ## @foo
	movl	4(%esp), %eax
	movapd	_b, %xmm0
	mulpd	LCPI1_0, %xmm0
	addpd	_a, %xmm0
	movapd	%xmm0, (%eax)
	ret	$4

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@84942 91177308-0d34-0410-b5e6-96231b3b80d8