[JVM IR] Use reference equality when comparing enums.

compiler/testData/codegen/box/enum/kt18731_2.kt

@@ -0,0 +1,14 @@
+// IGNORE_BACKEND_FIR: JVM_IR
+
+enum class Bar {
+    ONE {
+        override fun toString(): String {
+            if (this != TWO && this == ONE) return "OK" else return "FAIL"
+        }
+    },
+    TWO;
+}
+
+fun box(): String {
+    return Bar.ONE.toString()
+}