checkstl.cpp: small `LoopAnalyzer` cleanup #20106

Job	Run time
build_make (ubuntu:24.04)	4m 25s
build_make (alpine:3.23)	5m 46s
build_cmake (ubuntu:25.10, true, true)	5m 20s
build_cmake (ubuntu:24.04, true, true)	5m 19s
build_cmake (alpine:3.23, false, false)	4m 29s
build_make (ubuntu:25.10)	5m 3s
	30m 22s

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