`Logic.zeroExtend` should return `this` if `width` is the same as `newWidth`, and `Swizzle` optimization in SV generation

### Describe the bug

When zero-extending a signal, if there's nothing to zero-extend, we can just immediately return `this` rather than go through a `Swizzle`.

Additionally, when `Swizzle` gets only one signal with width >0, it shouldn't still add the `{}` around it.

### To Reproduce

- Call `zeroExtend` on a `Logic` with the `newWidth` equal to the `width`. Note that it creates a `swizzle` unnecessarily.
- Swizzle two signals, one of which is 0-width, and observe generated SV to contain `{}` around the one remaining signal.

### Expected behavior

_No response_

### Actual behavior

_No response_

### Additional: Dart SDK info

_No response_

### Additional: pubspec.yaml

_No response_

### Additional: Context

_No response_