[wasm] Fix printing of wasm prefixed opcode

Problem description:
For prefixed WASM opcode, opcode prefix is printed as Unknown, not the opcode itself.

Take v128.load as an example:
before fix                  ->  after fix
Unknown, 0x00, 0x04, 0x00,  ->  kExprS128LoadMem, 0x04, 0x00,

Change-Id: Id0cc5c723d19f60ad4f4f6c6ca338b5658c98c7e
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1694613
Commit-Queue: Jie Pan <jie.pan@intel.com>
Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#62697}

src/wasm/function-body-decoder.cc

@@ -151,7 +151,12 @@ bool PrintRawWasmCode(AccountingAllocator* allocator, const FunctionBody& body,
     unsigned length =
         WasmDecoder<Decoder::kNoValidate>::OpcodeLength(&decoder, i.pc());
 
+    unsigned offset = 1;
     WasmOpcode opcode = i.current();
+    if (WasmOpcodes::IsPrefixOpcode(opcode)) {
+      opcode = i.prefixed_opcode();
+      offset = 2;
+    }
     if (line_numbers) line_numbers->push_back(i.position());
     if (opcode == kExprElse || opcode == kExprCatch) {
       control_depth--;
@@ -188,7 +193,7 @@ bool PrintRawWasmCode(AccountingAllocator* allocator, const FunctionBody& body,
       }
 #undef CASE_LOCAL_TYPE
     } else {
-      for (unsigned j = 1; j < length; ++j) {
+      for (unsigned j = offset; j < length; ++j) {
         os << " 0x" << AsHex(i.pc()[j], 2) << ",";
       }
     }