checkstl.cpp: small `LoopAnalyzer` cleanup #20073

Job	Run time
build_make (ubuntu:24.04)	2m 55s
build_cmake (ubuntu:25.10, true, true)	2m 52s
build_make (alpine:3.23)	5m 50s
build_make (ubuntu:25.10)	3m 26s
build_cmake (alpine:3.23, false, false)	4m 28s
build_cmake (ubuntu:24.04, true, true)	2m 49s
	22m 20s

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