Fix backting if ending with operator part

Author: adpi2

compiler/src/dotty/tools/dotc/parsing/Parsers.scala

@@ -793,7 +793,7 @@ object Parsers {
         patchOver(source, Span(startClosing, endClosing), "")
         if colonRequired then
           // add backticks around operator and add colon
-          if lastSaved.isOperator then
+          if lastSaved.token == IDENTIFIER && Chars.isOperatorPart(lastSaved.name.last) then
             patch(Span(lastSaved.offset, lastSaved.offset + lastSaved.name.length), s"`${lastSaved.name}` :")
           else patch(Span(lastOffsetSaved), ":")
       else