[mips] Mark aggregate arguments passed in registers with the inreg attribute

Summary:
This allows us to easily identify them in the backend which in turn allows us
to handle them correctly for big-endian targets (where they must be shifted
into the upper bits of the register).

Depends on D5961

Reviewers: atanasyan

Reviewed By: atanasyan

Subscribers: cfe-commits, theraven

Differential Revision: http://reviews.llvm.org/D5962