This CL refactors the type-checking for br_table instructions.
Originally, we iterated over all targets of br_table and checked
if the values on the stack match the types expected by the
target's signature. However, this caused problems with type
checking unreachable br_table instructions where some stack
values are unavailable. According to the anyref proposal, the
expected type of br_table is the greatest lower bound of
all its targets. With the existing implementation, the expected
types were the types of the first target.

With this CL, we first calculate the expected types of br_table,
and only then inspect the stack if matching values are available.

R=titzer@chromium.org

Bug: v8:7581
Change-Id: I12208323bda88c363e28ffb0e002d59ef9a6b9d8
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1649791
Commit-Queue: Andreas Haas <ahaas@chromium.org>
Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#62354}