Copy register cache state before a conditional jump, and reuse it after.

[shattered]

This saves about 0.4% of cowfe size (168 bytes) on pdp11.

I think this is safe to apply to switch statements, since there's no fallthough?

[shattered]

This works even better with a stack, but there's weird interaction with nested functions (cache state from nested function leaks into parent?)

[davidgiven]

By the time cowbe is reached, any nested functions should be flattered (if I recall correctly, which I may not be).

I am suspicious about RegCacheRevert. I feel like it should be flushing the cache if the label doesn't match; otherwise, the stale cache from before the jump (which is modified to not flush the cache) will still be in effect, because E_jump_checkpoint didn't flush it.