Don't widen stable scrutinee types

Since ExprType are unstable, we can keep stable TermRef alone, in case
this provides more info to the pattern matcher/exhaustivity checker.

Author: Blaisorblade

compiler/src/dotty/tools/dotc/transform/PatternMatcher.scala

@@ -116,7 +116,7 @@ object PatternMatcher {
     /** Widen type as far as necessary so that it does not refer to a pattern-
      *  generated variable.
      */
-    private def sanitize(tp: Type): Type = tp.widenTermRefExpr match {
+    private def sanitize(tp: Type): Type = tp.widenIfUnstable match {
       case tp: TermRef if refersToInternal(false, tp) => sanitize(tp.underlying)
       case tp => tp
     }