Fix assertions generation for substituted members

compiler/testData/codegen/notNullAssertions/javaMultipleSubstitutions.java

@@ -0,0 +1,18 @@
+import org.jetbrains.annotations.NotNull;
+
+class A<T, U> {
+    @NotNull
+    T foo() { return null; }
+}
+
+class B<T> extends A<T, Integer> {
+    @Override
+    @NotNull
+    T foo() { return null; }
+}
+
+class C extends B<String> {
+    @Override
+    @NotNull
+    String foo() { return null; }
+}