Re-commit: [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

Like for the PowerPC target, I've had to add 'i' to the constraint mappings in
order to pass 2007-12-17-InvokeAsm.ll. It's not clear why 'i' has historically
been treated as a memory constraint.

llvm-svn: 232480