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_mir_transform/src/generator.rs

@@ -1235,7 +1235,7 @@ fn create_cases<'tcx>(
 
 impl<'tcx> MirPass<'tcx> for StateTransform {
     fn phase_change(&self) -> Option<MirPhase> {
-        Some(MirPhase::GeneratorLowering)
+        Some(MirPhase::GeneratorsLowered)
     }
 
     fn run_pass(&self, tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {