This patch tackles the problem of parsing Mips
register names in the standalone assembler llvm-mc.

Registers such as $A1 can represent either a 32 or
64 bit register based on the instruction using it.
In addition, based on the abi, $T0 can represent different
32 bit registers.


The problem is resolved by the Mips specific AsmParser
td definitions changing to work together. Many cases of
RegisterClass parameters are now RegisterOperand.


Contributer: Vladimir Medic