Decoding of gc/reference type instructions assumed that popping a value
from the stack would either throw an error or return a value of the
expected type. This is not true in unreachable contexts, where a
bottom-typed value can be returned.
This CL fixes this problem, adds tests which expose it, and improves
AddFunction() in the infrastructure of
function-body-decoder-unittest.cc.

Bug: v8:7748
Change-Id: I7e9d0caa9ba1687b68a5cdad7b99c054285d9f0e
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2440577
Commit-Queue: Manos Koukoutos <manoskouk@chromium.org>
Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#70246}