Fix two problems in SelectionDAGLegalize::ExpandBUILD_VECTOR's handling

Fix two problems in SelectionDAGLegalize::ExpandBUILD_VECTOR's handling
of BUILD_VECTORS that only have two unique elements:

1. The previous code was nondeterminstic, because it walked a map in
   SDOperand order, which isn't determinstic.
2. The previous code didn't handle the case when one element was undef
   very well.  Now we ensure that the generated shuffle mask has the
   undef vector on the RHS (instead of potentially being on the LHS)
   and that any elements that refer to it are themselves undef.  This
   allows us to compile CodeGen/X86/vec_set-9.ll into:

_test3:
	movd	%rdi, %xmm0
	punpcklqdq	%xmm0, %xmm0
	ret

instead of:

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

... saving a register.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@48060 91177308-0d34-0410-b5e6-96231b3b80d8