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/ExhaustiveWithNullabilityCheckBoolean.kt

@@ -0,0 +1,13 @@
+// KT-7857: when exhaustiveness does not take previous nullability checks into account
+fun foo(arg: Boolean?): Int {
+    if (arg != null) {
+        return when (<!DEBUG_INFO_SMARTCAST!>arg<!>) {
+            true -> 1
+            false -> 0
+            // else or null branch should not be required here!
+        }
+    } 
+    else {
+        return -1
+    }
+}