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.

tests/mini/aaequality.d

@@ -0,0 +1,48 @@
+void test(K,V)(K k1, V v1, K k2, V v2, K k3, V v3)
+{
+    V[K] a, b;
+    a[k1] = v1;
+    a[k2] = v2;
+    assert(a != b);
+    assert(b != a);
+    assert(a == a);
+    assert(b == b);
+    
+    b[k1] = v1;
+    assert(a != b);
+    assert(b != a);
+
+    b[k2] = v2;
+    assert(a == b);
+    assert(b == a);
+    
+    b[k1] = v2;
+    assert(a != b);
+    assert(b != a);
+    
+    b[k1] = v1;
+    b[k2] = v3;
+    assert(a != b);
+    assert(b != a);
+    
+    b[k2] = v2;
+    b[k3] = v3;
+    assert(a != b);
+    assert(b != a);
+}
+
+void main()
+{
+    test!(int,int)(1, 2, 3, 4, 5, 6);
+    test!(char[],int)("abc", 2, "def", 4, "geh", 6);
+    test!(int,char[])(1, "abc", 2, "def", 3, "geh");
+    test!(char[],char[])("123", "abc", "456", "def", "789", "geh");
+    
+    Object a = new Object, b = new Object, c = new Object;
+    test!(Object, Object)(a, a, b, b, c, c);
+    
+    int[int] a2 = [1:2, 2:3, 3:4];
+    int[int] b2 = [1:2, 2:5, 3:4];
+    int[int] c2 = [1:2, 2:3];
+    test!(int,int[int])(1,a2, 2,b2, 3,c2);
+}