[X86] AVX512: Specify compressed displacement for vmovntdqa

Use the max 64-bit element size with EVEX_CD8.  This should work since element
size is ignored for a full-vector access (FVM).
[X86] AVX512: Add non-temporal stores

Note that I followed the AVX2 convention here and didn't add LLVM intrinsics
for stores.  These can be generated with the nontemporal hint on LLVM IR
stores (see new test). The GCC builtins are lowered directly into nontemporal
stores.

<rdar://problem/17082571>