ARM64: always use i64 for the RHS of shift operations

Switching between i32 and i64 based on the LHS type is a good idea in
theory, but pre-legalisation uses i64 regardless of our choice,
leading to potential ISel errors.

Should fix PR19294.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@205519 91177308-0d34-0410-b5e6-96231b3b80d8