[CodeGen] Rename AtomicRMWExpansionKind to AtomicExpansionKind.

This lets us generalize its usage to the other atomic instructions.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@247428 91177308-0d34-0410-b5e6-96231b3b80d8
9 files changed