Teach SD some vector identities, allowing us to compile vec_set-9 into:

_test3:
	movd	%rdi, %xmm1
	#IMPLICIT_DEF %xmm0
	punpcklqdq	%xmm1, %xmm0
	ret

instead of:

_test3:
	#IMPLICIT_DEF %rax
	movd	%rax, %xmm0
	movd	%rdi, %xmm1
	punpcklqdq	%xmm1, %xmm0
	ret

This is still not ideal.  There is no reason to two xmm regs.

llvm-svn: 48058