checkstl.cpp: small `LoopAnalyzer` cleanup #20070

Job	Run time
build_cmake (ubuntu:25.10, true, true)	5m 9s
build_cmake (ubuntu:24.04, true, true)	2m 45s
build_make (alpine:3.23)	5m 47s
build_make (ubuntu:25.10)	4m 57s
build_make (ubuntu:24.04)	2m 31s
build_cmake (alpine:3.23, false, false)	4m 8s
	25m 17s

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