[mips][microMIPS] Implement disassembler support for 16-bit instructions

With the help of new method readInstruction16() two bytes are read and
decodeInstruction() is called with DecoderTableMicroMips16, if this fails
four bytes are read and decodeInstruction() is called with
DecoderTableMicroMips32.

Differential Revision: http://reviews.llvm.org/D6149