Prefixed opcodes have a 1 byte prefix, followed by LEB-encoded u32. This
changes all prefixed opcodes (gc, numeric, atomic), to that. (Simd was
already so.)

We can clean up read_prefix_opcode to return the total number of bytes,
1 byte prefix + leb encoded, that will be in a future patch.

Bug: v8:10810,v8:10994
Change-Id: Ia74604acc059c1336b87e9f477598732de219ca9
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2465057Reviewed-by: Manos Koukoutos <manoskouk@chromium.org>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Commit-Queue: Zhi An Ng <zhin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#70544}