[x86] use demanded bits to simplify masked store codegen

As noted in D52747, if we prefer IR to use trunc for bool vectors rather 
than and+icmp, we can expose codegen shortcomings as seen here with masked store.

Replace a hard-coded PCMPGT simplification with the more general demanded bits call
to improve things.

Differential Revision: https://reviews.llvm.org/D52964

llvm-svn: 344048