Functional change: Allow rtts as exception values.
Additional change: Remove liftoff subtyping TODO in anticipation of
removal of full ValueType usage in Liftoff.

Bug: v8:7748
Change-Id: I676a7fa6417d6e86bb148b4f5b9b086cc704928e
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2714702Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Commit-Queue: Manos Koukoutos <manoskouk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#72973}