SIMD opcodes consist of the prefix byte, then an LEB128 encoded int. We
were decoding this incorrectly as a fixed uint8. This fixes the decoder
to properly handle multi bytes.

In some cases, the multi byte logic is applied to all prefixed opcodes.
This is not a problem, since for values < 0x80, the LEB encoding is a
single byte, and decodes to the same int. If the prefix opcode has
instructions with index >= 0x80, it would be required to be LEB128
encoded anyway.

There are a bunch of trivial changes to test-run-wasm-simd, to change
the macro from BUILD to BUILD_V, the former only works for single byte
opcodes, the latter is a new template-based macro that correct handles
multi-byte opcodes. The only unchanged test is the shuffle fuzzer test,
which builds its own sequence of bytes without using the BUILD macro.

Bug: v8:10258
Change-Id: Ie7377e899a7eab97ecf28176fd908babc08d0f19
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2118476
Commit-Queue: Zhi An Ng <zhin@chromium.org>
Reviewed-by: Clemens Backes <clemensb@chromium.org>
Reviewed-by: Deepti Gandluri <gdeepti@chromium.org>
Cr-Commit-Position: refs/heads/master@{#67186}