[LLDB][MIPS] Fix Emulation of Compact branch and ADDIU instructions

Patch by Nitesh Jain.

This patch contains 2 changes:

  - Corrected target address calculation of compact branch instructions to reflect changes in disassembler (http://reviews.llvm.org/D17540).
  - Added emulation for (missing) 'Addiu' instruction.

Reviewers :jaydeep, bhushan, clayborg
Differential: http://reviews.llvm.org/D21064

llvm-svn: 273535