[arm] Distinguish the /U[qytnms]/, 'Uv', '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.

Of these, /U[qytnms]/ do not have backend tests but are accepted by clang.

No functional change intended.

Reviewers: t.p.northover

Reviewed By: t.p.northover

Subscribers: t.p.northover, aemerson, llvm-commits

Differential Revision: http://reviews.llvm.org/D8203

llvm-svn: 238921