ARM64: Refactor away a few redundant helpers.

The comment claimed that the register class information wasn't available
in the assembly parser, but that's not really true. It's just annoying to
get to. Replace the helper functions with references to the auto-generated
information.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@206802 91177308-0d34-0410-b5e6-96231b3b80d8