refactor(ngOptions): optimize getWatchables array

[sjbarker]

We need to prevent storing irrelevant or unnecessary watchers. See discussion in PR #11017:

@petebacondarwin wrote:

I realised that we can actually optimize this by only including things that are being specified. So for instance, if no disable when clause was provided we would not need to watch this expression. The same for the displayFn which computes the label.

[sjbarker]

closed in 3cea386