[ARM64] Print preferred aliases for SFBM/UBFM in InstPrinter

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@207219 91177308-0d34-0410-b5e6-96231b3b80d8
ARM64: fix assertion in ISelDAGToDAG

Also an unused variable, so double bonus!

This should deal with PR19548.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@207221 91177308-0d34-0410-b5e6-96231b3b80d8