[mips] [IAS] Add partial support for the ULHU pseudo-instruction.

Summary:
This only adds support for ULHU of an immediate address with/without a source register.
It does not include support for ULHU of the address of a symbol.

Reviewers: dsanders

Reviewed By: dsanders

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D9671

llvm-svn: 240410