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/ir/ir.tree/src/org/jetbrains/kotlin/ir/overrides/IrFakeOverrideBuilder.kt

@@ -378,7 +378,7 @@ class IrFakeOverrideBuilder(
         a: IrOverridableMember,
         b: IrOverridableMember
     ): Boolean {
-        return a > b
+        return a >= b
     }
 
     // Based on compareTo from FirOverrideService.kt
@@ -458,20 +458,12 @@ class IrFakeOverrideBuilder(
         }
         val candidates = mutableListOf<FakeOverride>()
         var transitivelyMostSpecific = overridables.first()
-        val transitivelyMostSpecificMember = transitivelyMostSpecific
         for (overridable in overridables) {
-            if (isMoreSpecificThenAllOf(overridable, overridables)
-            ) {
+            if (isMoreSpecificThenAllOf(overridable, overridables)) {
                 candidates.add(overridable)
             }
-            if (isMoreSpecific(
-                    overridable.override,
-                    transitivelyMostSpecificMember.override
-                )
-                && !isMoreSpecific(
-                    transitivelyMostSpecificMember.override,
-                    overridable.override
-                )
+            if (isMoreSpecific(overridable.override, transitivelyMostSpecific.override)
+                && !isMoreSpecific(transitivelyMostSpecific.override, overridable.override)
             ) {
                 transitivelyMostSpecific = overridable
             }
@@ -483,7 +475,7 @@ class IrFakeOverrideBuilder(
         }
         var firstNonFlexible: FakeOverride? = null
         for (candidate in candidates) {
-            if (candidate.override.returnType !is IrDynamicType) {
+            if (!candidate.override.returnType.isFlexible()) {
                 firstNonFlexible = candidate
                 break
             }