[FIR] Fix flattening of substitution overrides of intersection overrides for subsumed check

#KT-65555 Fixed

compiler/testData/diagnostics/tests/override/clashesOnInheritance/kt65555.kt

@@ -0,0 +1,8 @@
+// FIR_IDENTICAL
+// FULL_JDK
+// WITH_STDLIB
+// ISSUE: KT-65555
+
+interface MyCollection<E> : Collection<E>
+interface MyList<E> : MyCollection<E>, List<E>
+interface MyMutableList<E> : MyList<E>, MutableList<E>