This bug had possibly caused lots of grammar pollution which may need
addressing.
We want to process COME_FROMs to the same offset to be in *descending*
order so we have the larger range or biggest instruction interval
last. (I think they are sorted in increasing order, but for safety
we sort them). That way, specific COME_FROM tags will match up
properly. For example, a "loop" with an "if" nested in it should have
the "loop" tag last so the grammar rule matches that properly
Adjust Python 3 grammar for more COME_FROM -> COME_FROM_LOOP. And
remove optional COME_FROM_LOOP where possible. Previously, the
optional-ness was a result of inner nestings gobbling up the
COME_FROM.
We'll probably want to go back and fix this up in Python2.
