[RFC] PowerPC flags emulation optimization

https://github.com/delroth/dolphin/commit/81404494599f9d783c3a46edc44470c372ff9f90 explains in details what this is about.

Fairly nice performance boost for CPU emulation. @JMC47 reports a 5-12% performance improvement in CPU limited situations.

Caveats: currently broken for JITIL64, JITARM and JITILARM. Since it needs to touch interpreter code, it's difficult to isolate the change from these 3 JITs, and they will have to be modified.
I took a shot at this: https://github.com/magumagu/dolphin/compare/delroth:flags-opt...flags-opt .  I'll need to think a bit more about how to handle 64-bit values in JitIL; it's sort of outside the original design.  Oh, and there's a performance regression I haven't managed to track down yet.