Linear stepper behaving weird when going back a step

#### Bug, feature request, or proposal:
The stepper has weird styles when going back and leaving invalid steps, and it's inconsistent with the styles than when going back and leaving valid steps. It's also possible to have a combination of both where some labels will look disabled and some labels will look enabled when in fact they are all disabled.

#### What is the expected behavior?
On a stepper with 3 steps: A, B, C. If A, B and C are filled, valid and we are at C. When going back to B and making it invalid, it should mark C as disabled.
Then when going back to A and making it invalid too it should make B and C look disabled

#### What is the current behavior?
If B is made invalid, then A is made invalid too, B looks disabled but we can;t navigate there (because A is invalid), and C looks enabled even though we can't actually navigate there.

#### What are the steps to reproduce?
https://angular-qwtmdt.stackblitz.io

1. Fill requirements on Step 1.
2. Fill requirements on Step 2.
3. Fill requirements on Step 3.
4. Go back to Step 2 and erase a required field.
5. Go back to Step 1 and erase a required field.


#### What is the use-case or motivation for changing an existing behavior?
Behavior is inconsistent between steps 2 and 3. And Step 3 seems to promise that it's enabled when it's not, making the UI confusing.

Inizia con la riproduzione StackBlitz collegata e segui i passaggi elencati per confrontare le etichette dei passaggi con l'effettiva disponibilità della navigazione. Il lavoro è completato quando l'invalidazione di Step 2 e poi di Step 1 fa sì che ogni passaggio successivo non accessibile mostri sempre lo stile disabilitato.

Scritto dal modello di indicizzazione a partire dal testo della issue.