[SystemZ] Fix complex address matching when i128 is legal

Complex address matching currently handles truncations, under
the assumption that those are no-ops.  This is no longer true
when i128 is legal.  Change the code to only handle actual
no-op truncations.

Fixes https://github.com/llvm/llvm-project/issues/75708
Fixes https://github.com/llvm/llvm-project/issues/75714