J2K: Remove unnecessary casts of non-nullable expression to nullable type after conversion

#KT-11543 Fixed

j2k/testData/fileOrElement/nullability/notNullCast.java

@@ -0,0 +1,32 @@
+public class Passenger {
+    public static class PassParent {
+    }
+
+    public static class PassChild extends PassParent {
+    }
+
+    public PassParent provideNullable(int p) {
+        return p > 0 ? new PassChild() : null;
+    }
+
+    public void test1() {
+        PassParent pass = provideNullable(1);
+        assert pass != null;
+        accept1((PassChild) pass);
+    }
+
+    public void test2() {
+        PassParent pass = provideNullable(1);
+        if (1 == 2) {
+            assert pass != null;
+            accept2((PassChild) pass);
+        }
+        accept2((PassChild) pass);
+    }
+
+    public void accept1(PassChild p) {
+    }
+
+    public void accept2(PassChild p) {
+    }
+}