[mips][microMIPS] Extending size reduction pass with ADDIUSP and ADDIUR1SP

Author: milena.vujosevic.janicic
Reviewers: sdardis
The patch extends size reduction pass for MicroMIPS.
The following instructions are examined and transformed, if possible:
ADDIU instruction is transformed into 16-bit instruction ADDIUSP
ADDIU instruction is transformed into 16-bit instruction ADDIUR1SP
Differential Revision: https://reviews.llvm.org/D33887

llvm-svn: 305455