[mips] Small update to the logic behind the expansion of assembly pseudo instructions.

Summary:
The functions that do the expansion now return false on success and true otherwise. This is so
we can catch some errors during the expansion (e.g.: immediate too large). The next patch adds some test cases.

Reviewers: vmedic

Reviewed By: vmedic

Differential Revision: http://reviews.llvm.org/D4214