[mips] Fix passing of small structures for big-endian O32.

Summary:
Like N32/N64, they must be passed in the upper bits of the register.

The new code could be merged with the existing if-statements but I've
refrained from doing this since it will make porting the O32 implementation
to tablegen harder later.

Reviewers: vmedic

Reviewed By: vmedic

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D6463