IR: fix "most specific" check in IrFakeOverrideBuilder

There were several problems with it:

1) `isMoreSpecific` should return true if a == b. Otherwise
   `isMoreSpecificThenAllOf` would never return true because it's always
   invoked with a collection that contains the candidate. K1 behaves
   similarly, `OverridingUtil.isMoreSpecific` returns true if a == b.
   So in fact, "more" should be understood as "not less" here.
2) `transitivelyMostSpecificMember` in `selectMostSpecificMember` was
   always equal to the first element, so `isMoreSpecific` was invoked
   with incorrect arguments.
3) At the end of `selectMostSpecificMember`, we selected the first
   candidate with the non-flexible return type, however only dynamic
   type was considered. We need to check `isFlexible` via type system
   instead.

 #KT-66120 Fixed

compiler/testData/ir/irText/fakeOverrides/generics/intersectionWildcardsOverride.kt

@@ -1,8 +1,7 @@
+// FIR_IDENTICAL
 // TARGET_BACKEND: JVM
 // FULL_JDK
 // WITH_STDLIB
-// SEPARATE_SIGNATURE_DUMP_FOR_K2
-// ^ ISSUE: KT-66120
 
 // FILE: Java1.java
 import java.util.*;