make derivedFlexibleType reuse this when unchanged

Author: olhotak

compiler/src/dotty/tools/dotc/core/Types.scala

@@ -3365,7 +3365,7 @@ object Types {
     override def show(using Context) = "FlexibleType("+tp.show+")"
     def underlying(using Context) : Type = this.tp
     def derivedFlexibleType(under: Type)(using Context): Type =
-      FlexibleType(under)
+      if this.tp eq under then this else FlexibleType(under)
     override def computeHash(bs: Binders): Int = doHash(bs, tp)
     override def toString = "FlexibleType(%s)".format(tp)
     //override def hash = NotCached