[turbofan] Optimize silent hole checks on legacy const context slots.

test/mjsunit/asm/pointer-masking.js

+// Copyright 2015 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.
+
+var stdlib = this;
+var foreign = {};
+var heap = new ArrayBuffer(64 * 1024);
+
+
+var pm1 = (function(stdlib, foreign, heap) {
+  "use asm";
+  var HEAP8 = new stdlib.Int8Array(heap);
+  const MASK1 = 1023;
+  function load1(i) {
+    i = i|0;
+    var j = 0;
+    j = HEAP8[(i & MASK1)]|0;
+    return j|0;
+  }
+  function store1(i, j) {
+    i = i|0;
+    j = j|0;
+    HEAP8[(i & MASK1)] = j;
+  }
+  return {load1: load1, store1: store1};
+})(stdlib, foreign, heap);
+
+assertEquals(0, pm1.load1(0));
+assertEquals(0, pm1.load1(1025));
+pm1.store1(0, 1);
+pm1.store1(1025, 127);
+assertEquals(1, pm1.load1(0));
+assertEquals(1, pm1.load1(1024));
+assertEquals(127, pm1.load1(1));
+assertEquals(127, pm1.load1(1025));