valueflow.cpp: pass `ProgramMemory` by reference to `valueFlowForLoop2()` #19527

Job	Run time
build_cmake (alpine:3.23, false, false)	3m 57s
build_make (alpine:3.23)	6m 3s
build_make (ubuntu:24.04)	4m 10s
build_cmake (ubuntu:25.10, true, true)	5m 43s
build_cmake (ubuntu:24.04, true, true)	5m 11s
build_make (ubuntu:25.10)	5m 1s
	30m 5s

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