Is postosuchus a dinosaur

No, but it was very close. At the start of the Triassic, the infraclass Archosauromorpha started rapidly evolving to fill the niches left in the wake of the Permian Extinction. One group split into the crocodilians we all know and love, another became the dinosaurs. Postosuchus was a branch off of the first group, despite its similarities to the latter.