Test and implementation for KT-6819. Now when entry condition data flow information is used for subsequent conditions. #KT-6819 Fixed

compiler/testData/diagnostics/tests/smartCasts/varnotnull/kt6819.kt

@@ -0,0 +1,7 @@
+public fun test(o: String?): Boolean {
+    return when {
+        // Data flow info should propagate from o == null to o.length()
+        o == null, <!DEBUG_INFO_SMARTCAST!>o<!>.length() == 0 -> false 
+        else -> true
+    }
+}

compiler/testData/diagnostics/tests/smartCasts/varnotnull/kt6819.txt

@@ -0,0 +1,3 @@
+package
+
+public fun test(/*0*/ o: kotlin.String?): kotlin.Boolean