DropSkolemMap: simplify logic

No need to save the value of `refersToSkolem`: if it's true before we enter
`NamedType` it will be true after and `dropSkolem` will return `NoType`.

The previous logic could still be useful if we want to give more easily
actionable error messages in the future by only keeping in the type the skolems
we couldn't remove.

[Cherry-picked a1930c4]

Author: smarter

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

@@ -3531,33 +3531,30 @@ class MatchReducer(initctx: Context) extends TypeComparer(initctx) {
             class DropSkolemMap(skolem: SkolemType) extends TypeMap:
               var refersToSkolem = false
               def apply(tp: Type): Type =
+                if refersToSkolem then
+                  return tp
                 tp match
                   case `skolem` =>
                     refersToSkolem = true
                     tp
                   case tp: NamedType if betterMatchTypeExtractorsEnabled =>
-                    var savedRefersToSkolem = refersToSkolem
-                    refersToSkolem = false
-                    try
-                      val pre1 = apply(tp.prefix)
-                      if refersToSkolem then
-                        tp match
-                          case tp: TermRef => tp.info.widenExpr.dealias match
-                            case info: SingletonType =>
-                              refersToSkolem = false
-                              apply(info)
-                            case _ =>
-                              tp.derivedSelect(pre1)
-                          case tp: TypeRef => tp.info match
-                            case info: AliasingBounds =>
-                              refersToSkolem = false
-                              apply(info.alias)
-                            case _ =>
-                              tp.derivedSelect(pre1)
-                      else
-                        tp.derivedSelect(pre1)
-                    finally
-                      refersToSkolem |= savedRefersToSkolem
+                    val pre1 = apply(tp.prefix)
+                    if refersToSkolem then
+                      tp match
+                        case tp: TermRef => tp.info.widenExpr.dealias match
+                          case info: SingletonType =>
+                            refersToSkolem = false
+                            apply(info)
+                          case _ =>
+                            tp.derivedSelect(pre1)
+                        case tp: TypeRef => tp.info match
+                          case info: AliasingBounds =>
+                            refersToSkolem = false
+                            apply(info.alias)
+                          case _ =>
+                            tp.derivedSelect(pre1)
+                    else
+                      tp.derivedSelect(pre1)
                   case tp: LazyRef if betterMatchTypeExtractorsEnabled =>
                     // By default, TypeMap maps LazyRefs lazily. We need to
                     // force it for `refersToSkolem` to be correctly set.