[mips][microMIPS] Implement LWXS instruction.

Differential Revision: http://reviews.llvm.org/D5407


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@222348 91177308-0d34-0410-b5e6-96231b3b80d8
[mips][microMIPS] Add disassembler tests for new microMIPS 32-bit
instructions: LWXS, BGEZALS, BLTZALS, BEQZC, BNEZC, JALS and JALRS.

http://reviews.llvm.org/D5413


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@222349 91177308-0d34-0410-b5e6-96231b3b80d8