Emit prefix bytes when printing wasm code

This is useful when generating mjsunit tests from the fuzzer, since
without the prefix bytes, the opcodes are invalid.

This affects the json output for tracing turbofan, so the source view
in turbolizer will see this prefix as well.

Change-Id: Icd112e0d140c13f96530797da721650201e00660
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2006392Reviewed-by: Clemens Backes <clemensb@chromium.org>
Commit-Queue: Zhi An Ng <zhin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#65845}

src/wasm/function-body-decoder.cc

@@ -86,6 +86,17 @@ const char* RawOpcodeName(WasmOpcode opcode) {
   }
   return "Unknown";
 }
+const char* PrefixName(WasmOpcode prefix_opcode) {
+  switch (prefix_opcode) {
+#define DECLARE_PREFIX_CASE(name, opcode) \
+  case k##name##Prefix:                   \
+    return "k" #name "Prefix";
+    FOREACH_PREFIX(DECLARE_PREFIX_CASE)
+#undef DECLARE_PREFIX_CASE
+    default:
+      return "Unknown prefix";
+  }
+}
 }  // namespace
 
 bool PrintRawWasmCode(AccountingAllocator* allocator, const FunctionBody& body,
@@ -154,6 +165,7 @@ bool PrintRawWasmCode(AccountingAllocator* allocator, const FunctionBody& body,
     unsigned offset = 1;
     WasmOpcode opcode = i.current();
     if (WasmOpcodes::IsPrefixOpcode(opcode)) {
+      os << PrefixName(opcode) << ", ";
       opcode = i.prefixed_opcode();
       offset = 2;
     }