use data flow info in constraint system

intersect data flow value possible types
add intersection type (or exact type for the most cases) as a lower bound to constraint system

always get common super type for intersection type as a result
(avoid returning it, even in error messages)

compiler/testData/diagnostics/tests/smartCasts/inference/kt2746.kt

@@ -0,0 +1,19 @@
+//KT-2746 Do autocasts in inference
+import java.util.HashMap
+
+class C<T>(t :T)
+
+fun test1(a: Any) {
+    if (a is String) {
+        val <!UNUSED_VARIABLE!>c<!>: C<String> = C(a)
+    }
+}
+
+
+fun f<T>(t :T): C<T> = C(t)
+
+fun test2(a: Any) {
+    if (a is String) {
+        val <!UNUSED_VARIABLE!>c1<!>: C<String> = f(a)
+    }
+}