Reverted changes

• Revert "Drop phase.isTyper use in isLegalPrefix/asf" from Scala 3.6.4 #22653

Contributors

Thank you to all the contributors who made this release possible 🎉

According to git shortlog -sn --no-merges 3.6.4-RC1..3.6.4-RC2 these are:

     5  Wojciech Mazur