JVM_IR fix cast to not-null type

We can't rely on argument type nullability here, because it still can
hold an uninitialized value on JVM.

KT-50577 KT-35272 KT-27427

compiler/testData/codegen/box/casts/kt50577.kt

@@ -0,0 +1,26 @@
+abstract class A {
+    abstract val x: Any
+
+    init {
+        castX(this)
+    }
+}
+
+class B : A() {
+    override val x: Any = "abc"
+}
+
+fun castX(a: A) {
+    a.x as String
+}
+
+fun box(): String {
+    try {
+        B()
+    } catch (e: NullPointerException) {
+        return "OK"
+    } catch (e: ClassCastException) {
+        return "OK" // JS
+    }
+    return "Failed: should throw NPE"
+}