[X86] Make sure we don't remove sign extends of masks with AVX2 masked gathers.

We don't use k-registers and instead use the MSB so we need to make sure we sign extend the mask to the msb.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@319405 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed