The location of the immediates depend on the opcode length if a
s128.load/store was encoded using multiple bytes.

Bug: v8:10258
Change-Id: I09de8a37d442c0711de96ba4bb1746ae6732d83e
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2182960Reviewed-by: Clemens Backes <clemensb@chromium.org>
Commit-Queue: Zhi An Ng <zhin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#67623}