IR: deal with conflicting variances in computeSubstitutedSyntheticAccessor

compiler/testData/codegen/box/javaInterop/generics/propertyVarianceConflict.kt

@@ -0,0 +1,16 @@
+// TARGET_BACKEND: JVM
+
+// FILE: C.java
+
+class C<T> {
+    public C<? super T> getXx() { return this; }
+}
+
+// FILE: test.kt
+
+fun test() {
+    var c: C<out Any?> = C()
+    c = c.xx
+}
+
+fun box() = "OK"