The previous fix for this bug (crrev.com/c/1678365) pessimistically
would mark all shadowed variables as maybe_assigned. Unfortunately,
this doesn't work across a parse/preparse boundary, where the shadowing
variable is found via Scope::AnalyzePartially while the shadowed
variable is outside of the preparser entry point. In those cases, the
referencing proxy is copied to the outer scope, in which case the
dynamicness of the original lookup is lost and the maybe_assigned
pessimisation no longer applies.

This means that maybe_assigned status of a variable is dependent on
which function is being parsed. In particular, it can cause bytecode
to change on recompilation, causing issues for lazy source positions.

This patch allows SetMaybeAssigned to walk its shadowed variables,
and recursively set them to maybe_assigned too. Checking for
maybe_assigned changing prevents this recursion from having a
quadratic performance failure mode.

Bug: v8:8510
Bug: v8:9394
Change-Id: Id19fe1fad5ec8f0f9aa03b00eb24497f88f71216
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1677265
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Reviewed-by: Georg Neis <neis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#62458}