A class's fields can appear twice in the class AST, via the properties
array and the synthetised initializer method. This means that the
reindexer can end up visiting the same function literal twice, since the
T in AST is no longer a T but rather a DAG.

Now, we special case the class visitor in the reindexer to avoid these
double visits where appropriate. We know what kinds of fields can be
double visisted, so we don't need a visited set, but we now also have
one for debug builds to verify that each function is visited exactly
once.

Bug: chromium:974627
Change-Id: Ib531becc6e3f3c73f420b5fb49790fe4a2022d65
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1667003Reviewed-by: Sathya Gunasekaran <gsathya@chromium.org>
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Cr-Commit-Position: refs/heads/master@{#62282}