Split PL_padix into two variables

Split PL_padix into two variables

PL_padix keeps track of the position in the pad when pad_alloc has to
start scanning for an available slot.

The availability of a slot is determined differently for targets
(which may reuse slots that are already targets from previous state-
ments, at least when pad_reset is enabled) and constants (which may
not reuse targets).

Having the same index for both may require scanning the entire pad for
allocating a constant or GV.

t/re/uniprops.t was running far too slowly under USE_BROKEN_PAD_RESET
because of this.  pad_reset would reset PL_padix to point to the
beginning of a pad with a few hundred thousand entries.  pad_alloc
would then have to scan the entire pad before adding a GV to the end.

It is still too slow, even with this commit, but for other reasons.
(This is just a partial fix.)