[aarch64] Distinguish the 'Q' and 'm' inline assembly memory constraints.

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

Clang also has code for 'Ump', 'Utf', 'Usa', and 'Ush' but calls
llvm_unreachable() on this code path so they are not converted to a
constraint id at the moment.

No functional change intended.

Reviewers: t.p.northover

Subscribers: aemerson, llvm-commits

Differential Revision: http://reviews.llvm.org/D8177

llvm-svn: 232941