Fall back to interpreter for tw/twi if rA is a constant to avoid "a1 cannot be imm" error

Tw/twi is a "trap if comparison is true" instruction, which means that a cmp instruction is generated.
The emitter wants a register in the first argument to cmp, but constant propagation may try to pass in a constant instead.
For now, fall back to interpreter instead of displaying error: a proper way to translate this would be to load constant into a scratch register first like cmpXX does. 

Do games actually use these two instructions? I ran into this error when attempting to run Wii Linux.