The andi16, addiusp and jraddiusp micromips instructions were missing dedicated decoder methods in MipsDisassembler.cpp to properly decode immediate operands. These methods are added together with corresponding tests.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@223006 91177308-0d34-0410-b5e6-96231b3b80d8