Dotty

Drop phase.isTyper use in isLegalPrefix/asf #33203

Job	Run time
test	32m 8s
community_build_a	53m 50s
MiMa	11m 31s
test_non_bootstrapped	0s
test_scala2_library_tasty	0s
test_windows_fast	50m 47s
community_build_b	1h 18m 56s
community_build_c	1h 0m 56s
test_windows_full	0s
build-msi-package	0s
test_java8	0s
test_sbt	17m 42s
build-sdk-package / build	7m 53s
test-msi-package	0s
build-chocolatey-package / build	20s
test-chocolatey-package	0s
publish_nightly	0s
publish_release	0s
nightly_documentation	0s
open_issue_on_failure	0s
	5h 14m 3s

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