Allow merging of monomorphic accesses to tracked fields.

test/mjsunit/field-type-tracking.js

Index: test/mjsunit/field-type-tracking.js
diff --git a/test/mjsunit/field-type-tracking.js b/test/mjsunit/field-type-tracking.js
index 8da4ed56857378065eaaf1b87108ca470c9ac49d..b4901f34c34e706877316b65242dcfbaf75192d4 100644
--- a/test/mjsunit/field-type-tracking.js
+++ b/test/mjsunit/field-type-tracking.js
@@ -147,3 +147,21 @@
   baz(f3, {a: -1});
   assertUnoptimized(baz);
 })();
+
+(function() {
+  function Foo(x) { this.x = x; this.a = x; }
+  function Bar(x) { this.x = x; this.b = x; }
+  function readA(o) { return o.x.a; }
+  var f = new Foo({a:1});
+  var b = new Bar({a:2});
+  assertEquals(readA(f), 1);
+  assertEquals(readA(b), 2);
+  assertEquals(readA(f), 1);
+  assertEquals(readA(b), 2);
+  %OptimizeFunctionOnNextCall(readA);
+  assertEquals(readA(f), 1);
+  assertEquals(readA(b), 2);
+  assertOptimized(readA);
+  f.a.y = 0;
+  assertUnoptimized(readA);
+})();