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/tests/org/jetbrains/kotlin/codegen/BlackBoxCodegenTestGenerated.java

@@ -10552,6 +10552,11 @@ public class BlackBoxCodegenTestGenerated extends AbstractBlackBoxCodegenTest {
             runTest("compiler/testData/codegen/box/enum/ordinal.kt");
         }
 
+        @TestMetadata("refToThis.kt")
+        public void testRefToThis() throws Exception {
+            runTest("compiler/testData/codegen/box/enum/refToThis.kt");
+        }
+
         @TestMetadata("simple.kt")
         public void testSimple() throws Exception {
             runTest("compiler/testData/codegen/box/enum/simple.kt");