[X86] AVX512: Enable it in the Loop Vectorizer

This lets us experiment with 512-bit vectorization without passing
force-vector-width manually.

The code generated for a simple integer memset loop is properly vectorized.
Disassembly is still broken for it though :(.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@212634 91177308-0d34-0410-b5e6-96231b3b80d8