X86: Do splat promotion later, so the optimizer can chew on it first.

This catches many cases where we can emit a more efficient shuffle for a
specific mask or when the mask contains undefs. Once the splat is lowered to
unpacks we can't do that anymore.

There is a possibility of moving the promotion after pshufb matching, but I'm
not sure if pshufb with a mask loaded from memory is faster than 3 shuffles, so
I avoided that for now.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@173569 91177308-0d34-0410-b5e6-96231b3b80d8