RT #120446: /\Ga/ running slowly on long strings

This commit reverts my commit cf44e600505da0c8da2d64849647ce2d39c46808
(except for the tests), which incorrectly disabled fix-string intuiting
in the presence of anchored \G. I thought that the old behaviour was
logically incorrect, but it wasn't (or at least I don't see it that way
now, and none of the tests I added at the time fail under the old regime).