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.

从链接的 StackBlitz 复现开始，并按照列出的步骤将步骤标签与实际的导航可用性进行比较。当先使 Step 2 无效，再使 Step 1 无效后，每个无法访问的后续步骤都一致显示禁用样式时，工作即完成。

由索引模型根据 Issue 内容生成。