Compiler-rt part of D26230: Add (constant) masked load/store support

Summary:
Unfortunately, there is no way to emit an llvm masked load/store in
clang without optimizations, and AVX enabled. Unsure how we should go
about making sure this test only runs if it's possible to execute AVX
code.

Reviewers: kcc, RKSimon, pgousseau

Subscribers: kubabrecka, dberris, llvm-commits

Differential Revision: https://reviews.llvm.org/D26506

llvm-svn: 288162