ARM64: uncopy/paste helper function

It was doing functional but highly suspect operations on bools due to
the more limited shifting operands supported by memory instructions.

Should fix some MSVC warnings.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@205134 91177308-0d34-0410-b5e6-96231b3b80d8