rpeep(): elide just-nulled ops

rpeep(): elide just-nulled ops

Perl_rpeep() currently spots "empty" ops like OP_NULL, OP_SCOPE
and elides them from the op_next chain (by setting oldop->op_next to point
to the op following the current op). However, if rpeep() itself
op_null()'s the current op, then when the main loop is re-entered, that
mechanism is bypassed. Modify re-entry to the loop in this case so that
the just nulled op re-processed and thus elided.

(Also document what the OP_SASSIGN/OP_SUBSTR optimisation is doing;
studying that was what originally led me to this general fix.)