Clarify which kinds of MIR are allowed during which phases.

This enhances documentation with these details and extends the validator to check these requirements
more thoroughly. As a part of this, we add a new `Deaggregated` phase, and rename other phases so
that their names more naturally correspond to what they represent.

compiler/rustc_const_eval/src/transform/promote_consts.rs

@@ -42,7 +42,7 @@ pub struct PromoteTemps<'tcx> {
 
 impl<'tcx> MirPass<'tcx> for PromoteTemps<'tcx> {
     fn phase_change(&self) -> Option<MirPhase> {
-        Some(MirPhase::ConstPromotion)
+        Some(MirPhase::ConstsPromoted)
     }
 
     fn run_pass(&self, tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {