[mips] Support 9-bit offsets for the 'R' inline assembly memory constraint.

Summary:
The 'R' constraint is actually supposed to be much more complicated than
this and is defined in terms of whether it will cause macro expansion in
the assembler. 'R' is getting less useful due to architecture changes and
ought to be replaced by other constraints. We therefore implement 9-bit
offsets which will work for all subtargets and all instructions.

Reviewers: vkalintiris

Reviewed By: vkalintiris

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D8440

llvm-svn: 233537