[X86][MMX] Add support for MMX zero vector creation

As mentioned on PR35869, (and came up recently on D41517) we don't create a MMX zero register via the PXOR but instead perform a spill to stack from a XMM zero register.

This patch adds support for direct MMX zero vector creation and should make it easier to add better constant vector creation in the future as well.

Differential Revision: https://reviews.llvm.org/D41908

llvm-svn: 322525