[fuzzer] Switch code kind for multi-return fuzzer.

The code under test is handling of multi-return values in TurboFan and
hence actually independent of WebAssembly. The only reason to generate
WasmCode is in order to use the WebAseembly linkages. This changes the
generated code to have {STUB} kind instead of {WASM_FUNCTION} kind to
avoid having stack checks in the generated code which would require a
proper WasmInstanceObject to be allocated.

R=ahaas@chromium.org
BUG=chromium:862508

Change-Id: I4feb7bff1a42bbf59cfc5f249f2e0585ce7011ad
Reviewed-on: https://chromium-review.googlesource.com/1136438Reviewed-by: Andreas Haas <ahaas@chromium.org>
Commit-Queue: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#54436}

test/fuzzer/multi-return.cc

@@ -254,8 +254,7 @@ extern "C" int LLVMFuzzerTestOneInput(const uint8_t* data, size_t size) {
   }
   callee.Return(static_cast<int>(desc->ReturnCount()), returns.get());
 
-  OptimizedCompilationInfo info(ArrayVector("testing"), &zone,
-                                Code::WASM_FUNCTION);
+  OptimizedCompilationInfo info(ArrayVector("testing"), &zone, Code::STUB);
   Handle<Code> code = Pipeline::GenerateCodeForTesting(
                           &info, i_isolate, desc, callee.graph(),
                           AssemblerOptions::Default(i_isolate), callee.Export())