Fix a typo in comment

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@207499 91177308-0d34-0410-b5e6-96231b3b80d8
[ARM64]Fix a bug about incorrect operand order in an EXT instruction, which is introduced by r207485.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@207500 91177308-0d34-0410-b5e6-96231b3b80d8