Retain data flow info after when-expressions

This means the subject expression and the common data flow info of all entries'
conditions

compiler/testData/diagnostics/tests/dataFlowInfoTraversal/WhenEntry.kt

@@ -0,0 +1,24 @@
+fun foo(x: Number, y: Int) {
+    when (x) {
+        x as Int -> x : Int
+        y -> {}
+        else -> {}
+    }
+    <!TYPE_MISMATCH!>x<!> : Int
+}
+
+fun bar(x: Number) {
+    when (x) {
+        x as Int -> x : Int
+        else -> {}
+    }
+    x : Int
+}
+
+fun whenWithoutSubject(x: Number) {
+    when {
+        (x as Int) == 42 -> x : Int
+        else -> {}
+    }
+    x : Int
+}