[mips][microMIPS] Implement LBU16, LHU16, LW16, SB16, SH16 and SW16 instructions

Differential Revision: http://reviews.llvm.org/D5122


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@222653 91177308-0d34-0410-b5e6-96231b3b80d8