[llvm] [SystemZ] Add extended mnemonics tests. (PR #91733)

[Ulrich Weigand via llvm-commits]

https://github.com/uweigand commented:

Looks good to me, with a few minor comments inline.  I assume you verified that the GNU assembler agrees with these encodings as well?

https://github.com/llvm/llvm-project/pull/91733