Issue Description:

Adapted from https://twitter.com/steipete/status/1279030286584553472:

The code below produces a deprecation warning despite the fact that it never calls `foo` on OS versions where it is deprecated:

@available(iOS, deprecated: 13.0)
func foo() {}

func test() {
    if #available(iOS 13.0, *) { return }
    foo() // warning: 'foo' was deprecated in iOS 13
}

I expected the compiler to realize that control can never progress from within the body of the if statement to the call to `foo`, so the call can never be executed on iOS 13+. (The compiler is clearly able to do such control flow analysis, since it is part of how guard statements work.)

A workaround is to wrap the call in an else block:

func test2() {
    if #available(iOS 13.0, *) {
        return
    } else {
        foo() // No warning
    }
}