[systemz] Distinguish the 'Q', 'R', 'S', and 'T' inline assembly memory constraints.

Summary:
But still handle them the same way since I don't know how they differ on
this target.

No functional change intended.

Reviewers: uweigand

Reviewed By: uweigand

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D8251

llvm-svn: 232495