[wasm][liftoff] Correctly print prefixed opcodes in code comments

Prefixed opcodes do not get written correctly in code comments in
Liftoff. The reason is that only one byte is interpreted as the opcode,
but prefixed opcodes take two bytes. This CL loads the second byte if
necessary.

The change could be done in function-body-decoder-impl.h, but that could
lead to performance regressions: it is a hot code path, and the change
here is just for debugging.

R=clemensb@chromium.org

Bug: v8:10155
Change-Id: I2282c068c81b5b1e2e2ed9757f4e77687d1d4ede
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2091467Reviewed-by: Clemens Backes <clemensb@chromium.org>
Commit-Queue: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#66613}

src/wasm/baseline/liftoff-compiler.cc

@@ -661,8 +661,14 @@ class LiftoffCompiler {
       EmitBreakpoint(decoder);
     }
     TraceCacheState(decoder);
+#ifdef DEBUG
     SLOW_DCHECK(__ ValidateCacheState());
+    if (WasmOpcodes::IsPrefixOpcode(opcode)) {
+      byte op_index = *(decoder->pc() + 1);
+      opcode = static_cast<WasmOpcode>(opcode << 8 | op_index);
+    }
     DEBUG_CODE_COMMENT(WasmOpcodes::OpcodeName(opcode));
+#endif
   }
 
   void EmitBreakpoint(FullDecoder* decoder) {