DES MOINES, Iowa — Stung by paying billions of dollars for settlements and trials, chemical giant Bayer has been lobbying lawmakers in three states to pass bills providing it a legal shield from lawsuits that claim its popular weedkiller Roundup causes cancer.Nearly identical bills introduced in Iowa, Missouri and Idaho this year — with wording supplied by Bayer — would protect pesticide companies from claims they failed to warn that their product causes cancer, if their labels otherwise complied with the …