[regexp] Don't use eats_at_least for backwards loops

The eats_at_least (EAL) value is applied in forward-directions only.
Two reasons for that which are relevant to this CL:

- EAL's of neighboring nodes are combined additively, irrespective of
  their read_backward value.
- EatsAtLeastPropagator::VisitText uses the successor's
  eats_at_least_from_not_start value, which doesn't work properly for
  read_backwards successors (which may end at the start).

A symptom of this bug was that we applied an incorrect EAL of 255
starting at the initial 'x' of /x(?<=^x{4})/); for subject strings
shorter than 255 chars, this would result in an incorrect failure
result.

Bug: v8:11616
Change-Id: I4b2b1b78f0cea8f59e4beb1037ee46035d83c927
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2807596


Reviewed-by: Seth Brenith <seth.brenith@microsoft.com>
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#73848}