Use this function in both zend_mm_bitset_find_one and
zend_bitset_first.
Maybe zend_bitset.h is not quite the right place for it, but I did
not want to include this in a globally included header like
zend_long.h or zend_portability.h.
Previously the phi set was first computed during def propagation
and then computed again (per-block) during actual phi placement.
This commit changes this to store the phi set computed during
def propagation.
This makes SSA construction slightly faster (5%), but the main
purpose here is to pave the way for the next commit.
This also fixes a potential issue with the handling of irreducible
loops -- they generated additional phis, but these were not
accounted for in def propagation. (Though I'm not sure if we can
even have any irreducible loops right now.)
