[mips][ias] Implement .cpreturn directive.

Summary:
Based on a patch by David Chisnall. I've modified the original patch as follows:
* Moved the expansion to the TargetStreamers so that the directive isn't
  expanded when emitting assembly.
* Fixed an operand order bug.
* Changed the move instructions from DADDu to OR to match recent changes to GAS.

Reviewers: vkalintiris

Subscribers: llvm-commits, emaste, seanbruno, theraven

Differential Revision: http://reviews.llvm.org/D13017

llvm-svn: 248258