Finish implementing a readme entry: when inserting an i64 variable

Finish implementing a readme entry: when inserting an i64 variable
into a vector of zeros or undef, and when the top part is obviously
zero, we can just use movd + shuffle.  This allows us to compile
vec_set-B.ll into:

_test3:
	movl	$1234567, %eax
	andl	4(%esp), %eax
	movd	%eax, %xmm0
	ret

instead of:

_test3:
	subl	$28, %esp
	movl	$1234567, %eax
	andl	32(%esp), %eax
	movl	%eax, (%esp)
	movl	$0, 4(%esp)
	movq	(%esp), %xmm0
	addl	$28, %esp
	ret

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@48090 91177308-0d34-0410-b5e6-96231b3b80d8