Issue Description:

Putting this into "test.swift" and running "swiftc test.swift && ./test" takes many minutes to compile.

func read() -> String? {
return "hi"
}

func main() {
let x = read() ?? read() ?? read() ?? read() ?? read() ?? read() ?? read() ?? read() ?? read() ?? read() ?? read()
print(x!)
}

main()

However if you have only 6 of the read calls, for example, it takes seconds. 7 or more takes a very long time.

$ swiftc --version
Swift version 4.1-dev (LLVM 67b9a8e8c2, Clang e617c83242, Swift 4c99be4)
Target: x86_64-unknown-linux-gnu

$ lsb_release -a
No LSB modules are available.
Distributor ID: Ubuntu
Description: Ubuntu 16.04.3 LTS
Release: 16.04
Codename: xenial