Add a new method which enables one to change register classes.

See the Mips16ISetLowering.cpp patch to see a use of this.
For now now the extra code in Mips16ISetLowering.cpp is a nop but is
used for test purposes. Mips32 registers are setup and then removed and
then the Mips16 registers are setup. 

Normally you need to add register classes and then call
computeRegisterProperties.

llvm-svn: 177120