fix common assign issue on @a = (split(), 1)

fix common assign issue on @a = (split(), 1)

RT #127999 Slowdown in split + list assign

The compile-time common-value detection mechanism for OP_ASSIGN
was getting OP_SPLIT wrong.

It was assuming that OP_SPLIT was always dangerous. In fact,
OP_SPLIT is usually completely safe, not passing though any of its
arguments, except where the assign in (@a = split()) has been optimised
away and the array attached directly to the OP_SPLIT op, or the ops that
produce the array have been appended as an extra child of the OP_SPLIT op
(OPf_STACKED).