[SystemZ] Always use semantic instruction classes

[SystemZ] Always use semantic instruction classes

Define a couple of additional semantic classes and use them
throughout the .td files to make them more consistent and
more easily readable.

No functional change.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@286268 91177308-0d34-0410-b5e6-96231b3b80d8