Optimization: turn calls to UIDevice.CheckSystemVersion to a constant true value

An idea for a possible optimization in the linker:

Turn calls to UIKit.UIDevice.CurrentDevice.CheckSystemVersion (and the equivalent for macOS) to a constant true value, when a look at the deployment target proves it can never be false.

This will make such scenarios execute somewhat faster, and together with the dead code elimination optimization this should decrease app size somewhat too.