[turbofan] Fix merging of empty and non-empty state in load elimination.

test/mjsunit/compiler/regress-639210.js

+// Copyright 2016 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
+
+var m = (function m() {
+  "use asm"
+  var i32 = new Int32Array(4);
+  var f64 = new Float64Array(4);
+
+  function init() {
+    i32[0] = 1;
+    f64[0] = 0.1;
+  }
+
+  function load(b) {
+    return (b ? 0 : i32[0]) + i32[0];
+  }
+
+  function store(b) {
+    if (b|0) {
+    } else {
+      f64[0] = 42;
+    }
+    return f64[0];
+  }
+
+  return { init : init, load : load, store : store };
+})();
+
+m.init();
+
+%OptimizeFunctionOnNextCall(m.load);
+assertEquals(2, m.load());
+
+%OptimizeFunctionOnNextCall(m.store);
+assertEquals(0.1, m.store(1));