JVM_IR: fix for SingletonReferencesLowering

When replacing an enum entry reference with `this`, you need to take
`this` from the function's `dispatchReceiverParameter`, not the class's
`thisReceiver`. Otherwise the code generator fails to find the reference
among accessible variables.

compiler/testData/codegen/box/enum/refToThis.kt

@@ -0,0 +1,12 @@
+enum class Enum {
+    ENUM_VALUE {
+        override fun test() = ENUM_VALUE
+    };
+
+    abstract fun test(): Enum
+}
+
+fun box(): String {
+    if (Enum.ENUM_VALUE.test() != Enum.ENUM_VALUE) return "fail"
+    return "OK"
+}