Add the 16 bit version of addiu. To the assembler, the 16 and 32 bit are the

same so we put in the comment field an indicator when we think we are
emitting the 16 bit version. For the direct object emitter, the difference is 
important as well as for other passes which need an accurate count of 
program size. There will be other similar putbacks to this for various
instructions.

llvm-svn: 174747