[X86] Combine masked store and truncate into masked truncating stores.

We also need to combine to masked truncating with saturation stores, but I'm leaving that for a future patch.

This does regress some tests that used truncate wtih saturation followed by a masked store. Those now use a truncating store and use min/max to saturate.

Differential Revision: https://reviews.llvm.org/D57218

llvm-svn: 352230