[X86] Change the implementation of scalar masked load/store intrinsics to not...

[X86] Change the implementation of scalar masked load/store intrinsics to not use a 512-bit intermediate vector.

This is unnecessary for AVX512VL supporting CPUs like SKX. We can just emit a 128-bit masked load/store here no matter what. The backend will widen it to 512-bits on KNL CPUs.

Fixes the frontend portion of PR37386. Need to fix the backend to optimize the new sequences well.

llvm-svn: 331958