[mips] Mark aggregates returned in registers with the 'inreg' attribute.

Summary:
This allows us to easily find them in the backend after the aggregates have
been lowered to other types. This is important on big-endian targets using
the N32/N64 ABI's since these ABI's must shift small structures into the
upper bits of the register.

Reviewers: atanasyan

Reviewed By: atanasyan

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D5005

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@217160 91177308-0d34-0410-b5e6-96231b3b80d8