Optimize widen

For typer/*.scala, we had 3.4M calls to widen, of which
almost 3M were to the three cases that are now handled specially.

Author: odersky

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

@@ -1098,10 +1098,18 @@ object Types {
      *  def o: Outer
      *  <o.x.type>.widen = o.C
      */
-    final def widen(using Context): Type = widenSingleton match {
+    final def widen(using Context): Type = this match
+      case _: TypeRef | _: MethodOrPoly => this // fast path for most frequent cases
+      case tp: TermRef => // fast path for next most frequent case
+        if tp.isOverloaded then tp else tp.underlying.widen
+      case tp: SingletonType => tp.underlying.widen
       case tp: ExprType => tp.resultType.widen
-      case tp => tp
-    }
+      case tp =>
+        val tp1 = tp.stripTypeVar.stripAnnots
+        if tp1 eq tp then tp
+        else
+          val tp2 = tp1.widen
+          if tp2 ne tp1 then tp2 else tp
 
     /** Widen from singleton type to its underlying non-singleton
      *  base type by applying one or more `underlying` dereferences.