[wasm] Do the default int64-lowering for all non-i64 stores.

test/mjsunit/regress/wasm/regression-5884.js

+// Copyright 2017 the V8 project authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+load('test/mjsunit/wasm/wasm-constants.js');
+load('test/mjsunit/wasm/wasm-module-builder.js');
+
+(function() {
+  var builder = new WasmModuleBuilder();
+  builder.addMemory(31, 31, false);
+  builder.addFunction('test', kSig_i_iii)
+      .addBodyWithEnd([
+        // body:
+        kExprI64Const, 0x41, kExprI64Const, 0x41, kExprI64LtS, kExprI32Const,
+        0x01, kExprI32StoreMem, 0x00, 0x41, kExprUnreachable,
+        kExprEnd,  // @60
+      ])
+      .exportFunc();
+  var module = builder.instantiate();
+})();