Fix polymorphic keyed load handler selection for proxies.

test/mjsunit/regress/regress-crbug-603463.js

Index: test/mjsunit/regress/regress-crbug-603463.js
diff --git a/test/mjsunit/ignition/dead-code-source-position.js b/test/mjsunit/regress/regress-crbug-603463.js
similarity index 61%
copy from test/mjsunit/ignition/dead-code-source-position.js
copy to test/mjsunit/regress/regress-crbug-603463.js
index 95bb9183b880229a703e58503e9aec2a673015aa..20bfae65c54fa744d2470903fc92749f99726d67 100644
--- a/test/mjsunit/ignition/dead-code-source-position.js
+++ b/test/mjsunit/regress/regress-crbug-603463.js
@@ -2,8 +2,15 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+function load(a, i) {
+  return a[i];
+}
+
 function f() {
-  for (f(x) in []) { f(new f()) }
+  return load(new Proxy({}, {}), undefined);
 }
 
 f();
+f();
+load([11, 22, 33], 0);
+f();