When now tries to smart cast subject to not-null if contains no null case #KT-7857 Fixed

Some new tests + some fixed tests

compiler/testData/diagnostics/tests/when/ExhaustiveWithNullabilityCheck.kt

@@ -0,0 +1,14 @@
+// KT-7857: when exhaustiveness does not take previous nullability checks into account
+enum class X { A, B }
+fun foo(arg: X?): Int {
+    if (arg != null) {
+        return when (<!DEBUG_INFO_SMARTCAST!>arg<!>) {
+            X.A -> 1
+            X.B -> 2
+            // else or null branch should not be required here!
+        }
+    } 
+    else {
+        return 0
+    }
+}