checkstl.cpp: small `LoopAnalyzer` cleanup (#8659) #20184

Job	Run time
build_cmake (ubuntu:25.10, true, true)	2m 41s
build_make (alpine:3.23)	6m 3s
build_make (ubuntu:24.04)	2m 50s
build_cmake (ubuntu:24.04, true, true)	2m 46s
build_cmake (alpine:3.23, false, false)	4m 8s
build_make (ubuntu:25.10)	3m 5s
	21m 33s

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