Add test on data-flow

Idea is to intersect similar smartcasts on similar properties coming
from *different* instances (something like
'this.x == null && other.x == null' ).

It checks that we distinguish subjects of such smartcasts properly.

Currently behavior is undesired, see KT-27260

compiler/testData/diagnostics/tests/smartCasts/smartcastOnSameFieldOfDifferentInstances.kt

@@ -0,0 +1,11 @@
+// !DIAGNOSTICS: -UNUSED_EXPRESSION
+// See KT-27260
+
+class A(val x: String?) {
+    fun foo(other: A) {
+        when {
+            x == null && other.x == null -> "1"
+            <!DEBUG_INFO_SMARTCAST!>x<!>.length > 0 -> "2"
+        }
+    }
+}