[hexagon] Distinguish the 'o', 'v', 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.

No functional change intended.

Reviewers: kparzysz, adasgupt

Reviewed By: kparzysz, adasgupt

Subscribers: colinl, llvm-commits

Differential Revision: http://reviews.llvm.org/D8204

llvm-svn: 232374