JVM IR: Optimize field initializers in secondary constructors

compiler/testData/codegen/box/secondaryConstructors/fieldInitializerOptimization.kt

@@ -0,0 +1,20 @@
+// IGNORE_BACKEND: JS
+// IGNORE_BACKEND: JS_IR
+
+open class Base {
+    open fun setup() {}
+    init { setup() }
+}
+
+class Derived : Base {
+    constructor() : super()
+    override fun setup() { x = 1 }
+
+    // Technically, this field initializer comes after the superclass
+    // constructor is called. However, we optimize away field initializers
+    // which set fields to their default value, which is why x ends up with
+    // value 1 after the constructor call.
+    var x = 0
+}
+
+fun box(): String = if (Derived().x == 1) "OK" else "Fail"