perf(ngModel): avoid setting classes multiple times for parse / validator errors

[Narretz]

I'm submitting a ...

• bug report
• feature request
• other

Current behavior:

See #14292 (comment)

Expected / new behavior:

Minimal reproduction of the problem with instructions:

AngularJS version: 1.x.y

Browser: [all | Chrome XX | Firefox XX | Edge XX | IE XX | Safari XX | Mobile Chrome XX | Android X.X Web Browser | iOS XX Safari | iOS XX UIWebView | iOS XX WKWebView ]

Anything else: