Assertions are implemented with the new ASSERTION instruction.  The nfa
interpreter evaluates the assertion based on the current context in the
subject string every time a thread executes ASSERTION.  This is
analogous to what re2 and rust/regex do.

Alternatives to this approach:
- The interpreter could calculate eagerly for all assertion types
  whether they are satisfied whenever the current input position is
  advanced.  This would make evaluating the ASSERTION instruction itself
  cheaper, but at the cost of making every advance in the input string
  more expensive.  I suspect this would be slower on average because
  assertions are not that common that we typically evaluate >= 2
  assertions at every input position.
- Assertions in a regexp could be desugared into CONSUME_RANGE
  instructions, so that no new instruction would be necessary.  For
  example, the word boundary assertion \b is satisfied at a given
  position/state if we have just consumed a word character and will
  consume a non-word character next, or vice-versa.  The tricky part
  about this is that the assertion itself should not consume input, so
  we'd have to split (automaton) states according to whether we've
  arrived at them via a word character or not.  The current compiler is
  not really equipped for this kind of transformation.  For {start,end}
  of {line,file} assertions, we'd need to introduce dummy characters
  indicating start/end of input (say, 0x10000 and 0x10001) which we feed
  to the interpreter before respectively after the actual input.
  I suspect that this approach wouldn't make much of a difference for
  NFA execution. It would likely speed up (lazy) DFA execution though
  because assertions would be dealt with in the fast path.

Cq-Include-Trybots: luci.v8.try:v8_linux64_fyi_rel_ng
Bug: v8:10765
Change-Id: Ic2012c943e0ce54eb8662789fb3d4c1b6cd8d606
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2398644
Commit-Queue: Martin Bidlingmaier <mbid@google.com>
Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#70026}