RegExp assertions (e.g.: '^', '$', '\b', ...) sequences have certain
properties that this rewriter exploits:

1. They are zero-width and order-independent, thus one can remove all
duplicate assertions.
2. If a subsequence is guaranteed to fail, the entire sequence fails.
Any sequence always known to fail (e.g. containing both '\b' and '\B')
can be rewritten to a single node that triggers failure.

This CL generalizes the previous optimization for repeated assertions
to be order-independent, i.e. assertions only have to be in the same
sequence but not next to each other.

Bug: v8:6515, v8:6126
Change-Id: I3f92f081ce8a55ad8c34c269a09a6686e3b008f3
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1657925
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Peter Marshall <petermarshall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#62201}