Make == for associative arrays test for equality, not identity.

_aaEq was added to runtime/internal/aaA.d which forwards to
TypeInfo_AssociativeArray.equals in genobj.d. On the codegen side, DtoAAEquals
was added to gen/aa.cpp and is called from EqualExp::toElem in gen/toir.cpp.

I assume that the frontend will produce an error if == is used on associative
arrays of different type.

This fixes DMD bug 1429.

runtime/internal/aaA.d

@@ -705,6 +705,11 @@ body
 }
 
 
+int _aaEq(AA aa, AA ab, TypeInfo_AssociativeArray ti)
+{
+    return ti.equals(&aa, &ab);
+}
+
 /***********************************
  * Construct an associative array of type ti from
  * length pairs of key/value pairs.