[turbofan] Add missing test coverage for JSStrictEqual with symbols.

Bug: v8:6344, v8:8015
Change-Id: I6d96f039b47980f9df8d06c4097b70012fce3c82
Reviewed-on: https://chromium-review.googlesource.com/1215163Reviewed-by: Jaroslav Sevcik <jarin@chromium.org>
Commit-Queue: Benedikt Meurer <bmeurer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#55741}

test/mjsunit/compiler/strict-equal-symbol.js

+// Copyright 2018 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.
+
+// Flags: --allow-natives-syntax
+
+// Known symbols strict equality.
+(function() {
+  const a = Symbol("a");
+  const b = Symbol("b");
+
+  function foo() { return a === b; }
+
+  assertFalse(foo());
+  assertFalse(foo());
+  %OptimizeFunctionOnNextCall(foo);
+  assertFalse(foo());
+})();
+
+// Known symbol on one side strict equality.
+(function() {
+  const a = Symbol("a");
+  const b = Symbol("b");
+
+  function foo(a) { return a === b; }
+
+  assertTrue(foo(b));
+  assertFalse(foo(a));
+  assertTrue(foo(b));
+  assertFalse(foo(a));
+  %OptimizeFunctionOnNextCall(foo);
+  assertTrue(foo(b));
+  assertFalse(foo(a));
+})();
+
+// Feedback based symbol strict equality.
+(function() {
+  const a = Symbol("a");
+  const b = Symbol("b");
+
+  function foo(a, b) { return a === b; }
+
+  assertTrue(foo(b, b));
+  assertFalse(foo(a, b));
+  assertTrue(foo(a, a));
+  assertFalse(foo(b, a));
+  %OptimizeFunctionOnNextCall(foo);
+  assertTrue(foo(a, a));
+  assertFalse(foo(b, a));
+})();