JVM, JVM IR: Don't optimize null-checks based on nullability information

Since Java interop allows us to circumvent the Kotlin type system we
cannot rely on nullability information.

compiler/testData/codegen/box/platformTypes/unsafeNullCheck.kt

@@ -0,0 +1,18 @@
+// TARGET_BACKEND: JVM
+// FILE: Unsound.java
+
+import test.Wrap;
+
+public class Unsound {
+    public static <T> Wrap<T> get() {
+        return new Wrap<T>(null);
+    }
+}
+
+// FILE: 1.kt
+
+package test
+
+class Wrap<T>(val x: T)
+
+fun box(): String = if ((Unsound.get<String>() as Wrap<String>).x == null) "OK" else "Fail"