[SystemZ] Remove redundant frame MMOs

This fixes foldMemoryOperandImpl() so that it doesn't create duplicated
frame MMOs.  I hadn't realized when writing r185434 that it was the caller's
responsibility to add these.

No behavioural change intended.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@185704 91177308-0d34-0410-b5e6-96231b3b80d8