[x86] Distinguish the 'o', 'v', 'X', and 'i' 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, 'o' and 'v' are not tested but were already implemented.

I'm not sure why 'i' is required for X86 since it's supposed to be an
immediate constraint rather than a memory constraint. A test asserts
without it so I've included it for now.

No functional change intended.

Reviewers: nadav

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D8254

llvm-svn: 237517