When BranchElimination has to find the common prefix of a set of
BranchConditions in a Merge, it has to traverse a number of linked lists
of individual conditions, which is inefficient.
This CL improves its performance by grouping conditions between an
IfTrue/IfFalse and a Merge in a single entry of BranchConditions.
Additional change: Improve documentation of FunctionalList.

Change-Id: I93a58886151f6831cafb483aafb48e8e6c2433e5
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2917600
Commit-Queue: Manos Koukoutos <manoskouk@chromium.org>
Reviewed-by: Georg Neis <neis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#74793}