[fuzzer] Change the logic of GenerateOneOf

Change the logic behind GenerateOneOf so that not only
ref_null would appear almost everytime. Increase the probability
of other alternatives to appear as well.
Change the name of GenerateOptRef to GenerateRef.

Bug: v8:11954
Change-Id: I8bff1661c3d3a91105bca1ffa68e1e9b0b662240
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3229367
Commit-Queue: Maria Tîmbur <mtimbur@google.com>
Reviewed-by: Thibaud Michaud <thibaudm@chromium.org>
Reviewed-by: Manos Koukoutos <manoskouk@chromium.org>
Cr-Commit-Position: refs/heads/main@{#77438}