perf(input): prevent additional $digest when input is already touched

@kevinjamesus86 noticed that the input control would trigger a $digest
cycle every time it was blurred, https://github.com/angular/angular.js/commit/adcc5a00bf582d2b291c18e99093bb0854f7217c#commitcomment-7129512.

After the control is in a $touched state, other $digest cycles are
unnecesary.

Closes #8450