Move PERL_ASYNC_CHECK() from the runloop to control flow OPs.

Move PERL_ASYNC_CHECK() from the runloop to control flow OPs.

For the typical code this gives a 5% speedup, and removes the cost of "safe
signals". Tight looping code will show less gains, but should never be slower.

Subtle bugs might remain - there might be constructions that enter the runloop
(where signals used to be dispatched) but don't contain any PERL_ASYNC_CHECK()
calls themselves.