[MC] Use LShr for constant evaluation of ">>" on non-arm64 darwin.

Follow-up to r235963: this matches other assemblers and is less
unexpected (e.g. PR23227).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@252681 91177308-0d34-0410-b5e6-96231b3b80d8
5 files changed