The old version read the first element of the array into $top,
checked some values on it, and then either shifted the first
element into $frame, or shifted it off and discarded it.
So its cheaper to just shift it directly into $top, and then
use $top rather than $frame.
This change is mostly for FB internal organizational reasons.
Building is not effected beyond the fact that the target now
lands in hphp/hhvm/hhvm rather than src/hhvm/hhvm.
mwilliams