This fixes an invalid assumption when emitting code for matching '^'
(start of line) in multiline regexps and '\b', '\B' in general.

What we used to do: if the current trace's cp_offset (the offset from
the current position) was non-zero, we assumed that we were looking at
subject string index 1 or greater (i.e.: not at the start of the string
or before).

This is no longer valid since cp_offsets can now be negative.

This CL changes the logic to omit start- and bounds-checks only for
strictly positive cp_offsets, where the above assumption still holds.

Bug: chromium:996391
Change-Id: I79be4fc295c6f0b63e41c13d1e91fdd00f2f2b42
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1771794
Commit-Queue: Erik Corry <erikcorry@chromium.org>
Auto-Submit: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Erik Corry <erikcorry@chromium.org>
Cr-Commit-Position: refs/heads/master@{#63424}