In https://crrev.com/c/2084321 I added s128 load store to the fuzzer,
and updated the memop generator to use IsPrefixOpcode check. But it was
used wrongly. IsPrefixOpcode checks a 1 byte opcode and see if it is a
prefix opcode, but if memory_op is already a 2 byte opcode, it will fail
the IsPrefixOpcode check.

Bug: chromium:1059899
Change-Id: I4caadfb2feaf42ebb9f5578cb790ef8a1d08d173
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2095681Reviewed-by: Deepti Gandluri <gdeepti@chromium.org>
Commit-Queue: Zhi An Ng <zhin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#66638}