Quoting from the spec, the expected behavior for validating unreachable
code is that:

A polymorphic stack cannot underflow, but instead generates
Unknown types as needed.

(https://webassembly.github.io/spec/core/appendix/algorithm.html)

This CL changes the representation of the stack height in the
interpreter's side table builder from unsigned to signed to prevent
underflow, and makes some DCHECKs depend on code reachability.

R=clemensb@chromium.org

Bug: chromium:1017061
Change-Id: I4c999859019d6cefb76c1366ba0e98f199f7a0be
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1876813
Commit-Queue: Thibaud Michaud <thibaudm@chromium.org>
Reviewed-by: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#64546}