[Fir2Ir] Don't use FirSyntheticProperty as the cache key

^KT-64871

compiler/testData/codegen/box/multiplatform/k2/hashSetSize.kt

@@ -0,0 +1,20 @@
+// LANGUAGE: +MultiPlatformProjects
+// WITH_STDLIB
+// FULL_JDK
+// TARGET_BACKEND: JVM_IR
+
+// MODULE: common
+// FILE: common.kt
+
+val p = LinkedHashMap<Any, Any>()
+
+// MODULE: platform()()(common)
+// FILE: platform.kt
+
+val q = object : LinkedHashMap<Any, Any>() {
+    val s = size
+}
+
+fun box() : String {
+    return "OK"
+}

compiler/testData/codegen/box/multiplatform/k2/synteticProperty.kt

@@ -0,0 +1,32 @@
+// LANGUAGE: +MultiPlatformProjects
+// TARGET_BACKEND: JVM_IR
+
+// MODULE: lib
+// FILE: Derived.java
+
+public class Derived extends Base {
+    public String getFoo() { return "OK"; }
+}
+
+// FILE: Base.kt
+open class Base {
+    open val foo: String
+        get() = "FAIL"
+}
+
+
+// MODULE: common(lib)
+// FILE: common.kt
+
+val p = Derived().foo
+
+// MODULE: platform(lib)()(common)
+// FILE: platform.kt
+
+val q = object : Derived() {
+    val s = foo
+}
+
+fun box() : String {
+    return "OK"
+}