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

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

Reviewers: dsanders

Reviewed By: dsanders

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D9663

llvm-svn: 240782