Fix tests crashing under stricter pattern binding rules

Before this commit, the tests listed below would crash when the stricter
pattern binding rules were enforced (whether using `-source future` or
now enabled by default)

  - neg/parser-stability-14.scala
  - neg/i4453.scala
  - fuzzy/CCE-aafcaa9cd2611d22f63273738d637f5bec6e7152.scala
  - fuzzy/comment3.scala

with a common stack trace:

  dotty.tools.dotc.core.Types$PreviousErrorType cannot be cast to dotty.tools.dotc.core.Types$TermRef
  dotty.tools.dotc.transform.patmat.SpaceEngine$.isIrrefutable(Space.scala:316)
  dotty.tools.dotc.typer.Checking.recur$1(Checking.scala:828)
  dotty.tools.dotc.typer.Checking.checkIrrefutable(Checking.scala:843)
  dotty.tools.dotc.typer.Checking.checkIrrefutable$(Checking.scala:775)
  dotty.tools.dotc.typer.Typer.checkIrrefutable(Typer.scala:119)
    ...

compiler/src/dotty/tools/dotc/transform/patmat/Space.scala

@@ -312,8 +312,11 @@ object SpaceEngine {
    */
   def isIrrefutable(unapp: tpd.Tree, argLen: Int)(using Context): Boolean = {
     val fun1 = tpd.funPart(unapp)
-    val funRef = fun1.tpe.asInstanceOf[TermRef]
-    isIrrefutable(funRef, argLen)
+    fun1.tpe match
+      case funRef: TermRef =>
+        isIrrefutable(funRef, argLen)
+      case _ =>
+        false
   }
 }