This makes some checks a bit stricter to avoid accepting illegal relaxed
SIMD opcodes.

1) The default case in the Liftoff compiler should be UNREACHABLE,
   such that the switch case is required to cover all defined opcodes.
2) The {WasmOpcodes::IsRelaxedSimdOpcode} wrongly also returned {true}
   for opcodes like 0xfd300. We should really check nibbles 3-5 for the
   exact value 0xfd1.
3) {WasmOpcodes::Signature} was returning a non-null signatures for
   illegal opcodes like 0xfd200, because {IsRelaxedSimdOpcode} returned
   false, and then we would just use the lower bytes for the lookup in
   the SIMD signature table.

R=thibaudm@chromium.org
CC=gdeepti@chromium.org

Bug: chromium:1324081
Change-Id: Idbfde570ccd782e59b47b96e7ca8cc28fa7fae98
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3687309
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Reviewed-by: Thibaud Michaud <thibaudm@chromium.org>
Cr-Commit-Position: refs/heads/main@{#80934}