[llvm] [SystemZ] Removes string from the addressing mode classes (PR #215643)

[Ulrich Weigand via llvm-commits]

https://github.com/uweigand commented:

The title of this PR is a bit misleading - this is really the first step in adding 31-bit codegen support, so that should be obvious from the title.  Also, it would be good to see a bit more of how 31-bit support will look as a complete solution in order to make more informed decisions.

https://github.com/llvm/llvm-project/pull/215643