Jit64 RegCache cleanup

Mostly a small cleanup of the Jit64 RegCache + a way to not flush the RegCache on conditional branches.
While I somewhat agree that it should be self-explanatory, thinking of an enum name for example will put us in the same situation - if we knew a name, it would either be ```FlushAndDoStuff()``` or ```Flush(AndDoStuff)``` (just like ```Flush(FLUSH_ALL)``` did before).
Well, how about ```FlushAndClearState()```?