Rename mattr names for AVX-512 to from avx-512 -> avx512f, avx-512-pfi ->...

Rename mattr names for AVX-512 to from avx-512 -> avx512f, avx-512-pfi -> av512pf, avx-512-cdi -> avx512cd, avx-512-eri->avx512er. This matches better with official docs and what gcc patches appearto be using. I didn't touch the has* functions or the feature flag names to avoid change the td and lowering file while commits are still happening.

llvm-svn: 188859