[X86][SSE] Lower scalar_to_vector(0) to zero vector

Replaces an xor+movd/movq with an xorps which will be shorter in codesize, avoid an int-fpu transfer, allow modern cores to fast path the result during decode and helps other combines recognise an all-zero vector.

The only reason I can think of that we'd want to keep scalar_to_vector in this case is to help recognise the upper elts are undef but this doesn't seem to be a problem.

Differential Revision: https://reviews.llvm.org/D29097

llvm-svn: 293438