[wasm][fuzzer] Fix fuzzer for saturated conversion

Opcodes for saturated conversions are 2 bytes long, hence EmitWithPrefix is necessary.

Bug: chromium:1084344
Change-Id: I7b8821ba952abee6b113f705559f2f3572d0f7c4
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2209058
Commit-Queue: Richard Stotz <rstz@chromium.org>
Reviewed-by: Thibaud Michaud <thibaudm@chromium.org>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#67918}