This patch adds the register class for MIPS16 as well as the ability for

llc to recognize MIPS16 as a MIPS ASE extension. -mips16 will mean the
mips16 ASE for mips32 by default.

As part of fixing of adding this we discovered some small changes that
need to be made to MipsInstrInfo::storeRegToStackSLot and
MipsInstrInfo::loadRegFromStackSlot. We were using some "==" equality tests
where in fact we should have been using Mips::<regclas>.hasSubClassEQ instead,
per suggestion of Jakob Stoklund Olesen.

Patch by Reed Kotler.

llvm-svn: 156958