Performance problems with Swift 4.2

[Joe_Groff]

@nuclearace

I would hope that as time goes on this kind of optimization can be picked up more reliably by the compiler. I would not expect most Swift programmers to understand that doing this would lead more performant code.

The optimizer continues to improve its understanding of exclusivity checks, and further improvements should be coming in Swift 5, enough to make us feel comfortable enabling them more aggressively—see Enabling run-time exclusivity checking in release mode for details.