[X86][AVX512] Tag GATHER/SCATTER instruction scheduler classes

NOTE: At the moment these use the WriteLoad/WriteStore classes, which severely underestimates the costs. This needs to be reviewed.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@319829 91177308-0d34-0410-b5e6-96231b3b80d8
3 files changed