v128.const (kExprS128) is the only constant expression supported
(similar to the other value types).

Bug: v8:10731
Change-Id: I9b11b47a851903dfd79d3590eff67b615057f81c
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2308389
Commit-Queue: Zhi An Ng <zhin@chromium.org>
Reviewed-by: Bill Budge <bbudge@chromium.org>
Reviewed-by: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#68975}