Before dd6f4d4f, this function was reading a single u8 and verifying
that it was 0x00. The referenced commit dropped this check as part of
implementing decoding for table.get, but neglected to change the read type
to u32v, which is the specified type of this immediate. Fix that.

R=ahaas@chromium.org, titzer@chromium.org

Change-Id: Ic2ce795023ec57be2c95aa79e62d3ccd1aa9c43c
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1747178
Commit-Queue: Andreas Haas <ahaas@chromium.org>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#63281}