Now tracking constant values for most operations.
This will make it easier to enable memory access optimizations and hacks.
Not all operations are updating the constant registers, only those that
are likely to participate in constant value building. Constant values are
cleared on every basic block, though this may be too zealous it prevents
weird fall-through cases.
Adding basic constant checks and updates to memory instructions.
Only those instructions likely to have immediate values have been updated.
It'd be nice to go further than this and a) centralize the ea calculations
b) move this all to the analysis pass c) generate mov's in Read/WriteMemory
using the immediates as bases instead of all the register work.

Also fixing a broken stwcx that was causing crashes.