[mips][microMIPS] Extending size reduction pass with LBU16, LHU16, SB16 and SH16

Author: milena.vujosevic.janicic
Reviewers: sdardis
The patch extends size reduction pass for MicroMIPS.
The following instructions are examined and transformed, if possible:
LBU instruction is transformed into 16-bit instruction LBU16
LHU instruction is transformed into 16-bit instruction LHU16
SB instruction is transformed into 16-bit instruction SB16
SH instruction is transformed into 16-bit instruction SH16
Differential Revision: https://reviews.llvm.org/D33091

llvm-svn: 304550