The motivating example is JetStream 2's UniPoker test, which tests
whether a sorted string of Unicode playing cards contains a five-card
straight using a regular expression. In the top-level generated loop for
this RegExp, we see this loop exit condition:

00000350000C2067    27  83fffe         cmpl rdi,0xfe
00000350000C206A    2a  0f8da8e40000   jge 00000350000D0518  <+0xe4d8>

Meaning if the current position is pointing at the very last (16-bit)
character, then we exit the loop. Otherwise we go on and try to find
various matches starting at the current position. However, we can see
in the original expression that any possible match is at least 10
characters (5 astral-plane Unicode values), so we're wasting a lot of
time attempting to find matches in cases where we're too close to the
end of the string for any match to succeed.

This example might be a bit contrived, but I expect that an improvement
in this bounds check would help a larger family of regular expressions,
where the minimum match length is large relative to the string being
matched and we don't meet the other necessary criteria for fast Boyer-
Moore lookahead.

To get the desired bounds check in this case, this patch does the
following:
1. Compute accurate EatsAtLeast values for every node during the
   analysis phase. This could end up doing more work than the current
   implementation, but analysis already has to touch every node, so it
   seems like a cache-friendly time to compute these values. In some
   cases, this might be less total work than the current implementation,
   because the current implementation might recompute the same node
   multiple times.
2. When emitting a quick check, use the EatsAtLeast value from the
   predecessor ChoiceNode for the bounds check.

This improves the UniPoker score on my machine by about 4%, because it
cuts the time spent checking for straights roughly in half, and checking
for straights originally accounted for about 8% of the total time.

Bug: v8:9305
Change-Id: I110b190c2578f73b2263259d5aa5750e921b01be
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1702125
Commit-Queue: Seth Brenith <seth.brenith@microsoft.com>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#62919}