[NI] Fix assertion error at FlexibleTypeImpl

Use KotlinTypeFactory for FlexibleType creation

compiler/resolution/src/org/jetbrains/kotlin/resolve/calls/NewCommonSuperTypeCalculator.kt

@@ -53,7 +53,7 @@ object NewCommonSuperTypeCalculator {
         if (!thereIsFlexibleTypes) return lowerSuperType
 
         val upperSuperType = commonSuperTypeForSimpleTypes(types.map { it.upperIfFlexible() }, depth)
-        return FlexibleTypeImpl(lowerSuperType, upperSuperType)
+        return KotlinTypeFactory.flexibleType(lowerSuperType, upperSuperType)
     }
 
     private fun commonSuperTypeForSimpleTypes(types: List<SimpleType>, depth: Int): SimpleType {

compiler/resolution/src/org/jetbrains/kotlin/resolve/calls/inference/components/NewTypeSubstitutor.kt

@@ -46,7 +46,7 @@ interface NewTypeSubstitutor {
                     }
                     else {
                         // todo discuss lowerIfFlexible and upperIfFlexible
-                        FlexibleTypeImpl(lowerBound?.lowerIfFlexible() ?: type.lowerBound, upperBound?.upperIfFlexible() ?: type.upperBound)
+                        KotlinTypeFactory.flexibleType(lowerBound?.lowerIfFlexible() ?: type.lowerBound, upperBound?.upperIfFlexible() ?: type.upperBound)
                     }
                 }
             }

compiler/resolution/src/org/jetbrains/kotlin/types/TypeApproximator.kt

@@ -152,7 +152,7 @@ class TypeApproximator {
                      *
                      * If U_1 <: U_2.lower .. U_2.upper, then we know only that U_1 <: U_2.upper.
                      */
-                    return FlexibleTypeImpl(lowerResult?.lowerIfFlexible() ?: type.lowerBound,
+                    return KotlinTypeFactory.flexibleType(lowerResult?.lowerIfFlexible() ?: type.lowerBound,
                                             upperResult?.upperIfFlexible() ?: type.upperBound)
                 }
                 else {