[SystemZ] Define the return instruction as a pseudo alias of BR

This is the first of a few patches to reduce the dupliation of encoding
information.  The return instruction is a normal BR in which one of the
registers is fixed.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@191364 91177308-0d34-0410-b5e6-96231b3b80d8