[x86] eliminate unnecessary vector compare for AVX masked store

The masked store instruction only cares about the sign-bit of each mask element,
so the compare s<0 isn't needed.

As noted in PR11210:
https://bugs.llvm.org/show_bug.cgi?id=11210
...fixing this should allow us to eliminate x86-specific masked store intrinsics in IR.
(Although more testing will be needed to confirm that.)

I filed a bug to track improvements for AVX512:
https://bugs.llvm.org/show_bug.cgi?id=34584

Differential Revision: https://reviews.llvm.org/D37446

llvm-svn: 313089