op.c:newPADOP: Allocate GV pad slots like constants

op.c:newPADOP: Allocate GV pad slots like constants

Requesting a pad slot for a constant (and GVs in pads are like con-
stants, in that the pad slot will always hold that same value, which
is shared in recursion) will instruct pad_alloc to avoid anything
marked PADTMP; i.e., a  slot used as a target by a previous statement.

Without this, under -Accflags=-DUSE_BROKEN_PAD_RESET and threads,
configpm fails an assertion, because a concat operator tries to do
sv_grow on its target, but that target has ended up becoming a GV.

With this commit, all tests pass, but t/re/uniprops.t takes ‘forever’
(cf. commits 325e1816 and 7db6405c, which caused and fixed, respec-
tively, a similar bug).