simplifier: bring back some optimizations while avoiding contradiction
simplifier: bring back some optimizations while avoiding contradiction

c.f. e2d2d619cb2b3916f67bcff8eb7bc21b6a8b9cb7

FStarLang/FStar,FStarLang/FStar,FStarLang/FStar,A-Manning/FStar,A-Manning/FStar,A-Manning/FStar