If there was an assignment to a maybe-shadowing dynamic variable,
then the shadowing variable would be marked maybe_assigned, but the
maybe-shadowed variable would stay unchanged. This meant that in
non-shadowing cases, the not-actually-shadowed variable would have
the wrong maybe_assigned state, and e.g. would break context
specialization.

This patch pessimistically unconditionally sets maybe_assigned on
variables shadowed by a dynamic variable in a `with` scope. This
marking can cause false positives and sub-optimal optimization for
some functions with 'with' blocks, but it's also the simplest fix
for this issue which doesn't affect performance in the common case
of no 'with' blocks.

Bug: v8:9394
Change-Id: I6924bd7d48dda61232aa9d72c39df1c76c665c67
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1678365
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Auto-Submit: Leszek Swirski <leszeks@chromium.org>
Reviewed-by: Georg Neis <neis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#62407}