Avoid templates, just encode all wasm opcodes as 2-byte LEB instead.

R=zhin@chromium.org

Bug: v8:10258
Change-Id: I3bfd5235b235a5d9366e0007e915a2c02a09b0d4
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2182638Reviewed-by: Zhi An Ng <zhin@chromium.org>
Reviewed-by: Deepti Gandluri <gdeepti@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#67625}