To clarify: that's more of a problem in programming the GPU in a portable manner, and that may or may not turn into a problem depending on the implementation. GPUs can do loops that run a different number of times per thread just fine, at the cost of having to wait for the slowest to finish. Things start to turn sideways if you want diverging branches though (at least AMD kind of has support for this, but it's limited). If you want animations to work there's a good chance your search would become pretty complicated and you run the risk of smacking into that.

Also, I doubt GPU vendors would add special-purpose hardware like that. They'd much rather just add some new shader instructions to help doing it.