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/ExhaustiveWithNullabilityCheckBefore.kt

@@ -0,0 +1,12 @@
+// 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 0
+    }
+    return when (<!DEBUG_INFO_SMARTCAST!>arg<!>) {
+        X.A -> 1
+        X.B -> 2
+        // else or null branch should not be required here!
+    }
+}