[SystemZ] Fix caller-allocated save slot FIXME

Get rid of some old code (and associated FIXME) for handling the
caller-allocated register save area.  No behavioural change intended.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@185525 91177308-0d34-0410-b5e6-96231b3b80d8