[SystemZ] Add CodeGen test cases

This adds all CodeGen tests for the SystemZ target.

This version of the patch incorporates feedback from a review by
Sean Silva.  Thanks to all reviewers!

Patch by Richard Sandiford.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@181204 91177308-0d34-0410-b5e6-96231b3b80d8