[turbofan] Only infer signedness for Float64->Word32 representation change from the input type.

test/mjsunit/compiler/regress-f64-w32-change.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.
 
 // Flags: --allow-natives-syntax
 
-function get_thrower() {
-  "use strict";
-  return Object.getOwnPropertyDescriptor(arguments, "callee").get;
-}
+var f = (function () {
+  "use asm";
+  var f64use = 0;
+  function f(x, b) {
+    x = x|0;
+    b = b >>> 0;
+    var f64 = x ? -1 : b;
+    f64use = f64 + 0.5;
+    var w32 = x ? 1 : f64;
+    return (w32 + 1)|0;
+  }
 
-var f = (function(v) {
-  "use asm";
-  function fun() {
-    switch (v) {}
-  }
-  return {
-    fun: fun
-  };
-})(get_thrower()).fun;
+  return f;
+})();
 
 %OptimizeFunctionOnNextCall(f);
-f();
+assertEquals(0, f(0, -1));