[mips][ias] Implement ulh macro.

Summary:
This macro is needed to prevent test/CodeGen/Mips/2008-08-01-AsmInline.ll from
failing after the integrated assembler is enabled by default.

Reviewers: vkalintiris

Subscribers: llvm-commits, dsanders

Differential Revision: http://reviews.llvm.org/D13654

llvm-svn: 250414