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"
+        }
+    }
+}

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

@@ -0,0 +1,10 @@
+package
+
+public final class A {
+    public constructor A(/*0*/ x: kotlin.String?)
+    public final val x: kotlin.String?
+    public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public final fun foo(/*0*/ other: A): kotlin.Unit
+    public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+}

compiler/tests/org/jetbrains/kotlin/checkers/DiagnosticsTestGenerated.java

@@ -19377,6 +19377,11 @@ public class DiagnosticsTestGenerated extends AbstractDiagnosticsTest {
                 runTest("compiler/testData/diagnostics/tests/smartCasts/smartCastOnWhen.kt");
             }
 
+            @TestMetadata("smartcastOnSameFieldOfDifferentInstances.kt")
+            public void testSmartcastOnSameFieldOfDifferentInstances() throws Exception {
+                runTest("compiler/testData/diagnostics/tests/smartCasts/smartcastOnSameFieldOfDifferentInstances.kt");
+            }
+
             @TestMetadata("thisWithLabel.kt")
             public void testThisWithLabel() throws Exception {
                 runTest("compiler/testData/diagnostics/tests/smartCasts/thisWithLabel.kt");

compiler/tests/org/jetbrains/kotlin/checkers/javac/DiagnosticsUsingJavacTestGenerated.java

@@ -19377,6 +19377,11 @@ public class DiagnosticsUsingJavacTestGenerated extends AbstractDiagnosticsUsing
                 runTest("compiler/testData/diagnostics/tests/smartCasts/smartCastOnWhen.kt");
             }
 
+            @TestMetadata("smartcastOnSameFieldOfDifferentInstances.kt")
+            public void testSmartcastOnSameFieldOfDifferentInstances() throws Exception {
+                runTest("compiler/testData/diagnostics/tests/smartCasts/smartcastOnSameFieldOfDifferentInstances.kt");
+            }
+
             @TestMetadata("thisWithLabel.kt")
             public void testThisWithLabel() throws Exception {
                 runTest("compiler/testData/diagnostics/tests/smartCasts/thisWithLabel.kt");