[mips][microMIPS] Implement CodeGen support for 16-bit instruction ADDIUR2.

Differential Revision: http://reviews.llvm.org/D5800


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@222352 91177308-0d34-0410-b5e6-96231b3b80d8