Backends do not care about the concrete type, they only need to know the
"kind" (e.g. "ref" or "i32").
In order to prepare Liftoff to use the value kind instead of the
value type for all stored data, this CL moves the kind out of the
ValueType and makes it a top-level enum.

R=manoskouk@chromium.org

Bug: v8:11477
Change-Id: I489d6c5207e6ff1b66e2afbe78a156d66df27eb3
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2707169
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Reviewed-by: Manos Koukoutos <manoskouk@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#72896}