[mips] [IAS] Unify common functionality of LA and LI.

Summary: A side-effect of this is that LA gains proper handling of unsigned and positive signed 16-bit immediates and more accurate error messages.

Reviewers: dsanders

Reviewed By: dsanders

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D9290

llvm-svn: 237255