Problem description:
For prefixed WASM opcode, opcode prefix is printed as Unknown, not the opcode itself.

Take v128.load as an example:
before fix                  ->  after fix
Unknown, 0x00, 0x04, 0x00,  ->  kExprS128LoadMem, 0x04, 0x00,

Change-Id: Id0cc5c723d19f60ad4f4f6c6ca338b5658c98c7e
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1694613
Commit-Queue: Jie Pan <jie.pan@intel.com>
Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#62697}