Changes:
- Implement subtyping for eqref.
- (Driveby) Declare more functions as constexpr in ValueType.
- Make minor changes needed to handle ref.eq.
- Write an elementary test.

Bug: v8:7748
Change-Id: I11d54227798ce56de70f3a6f83305b2f80b2f57f
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2193715
Commit-Queue: Manos Koukoutos <manoskouk@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#67752}