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Issue Description:

protocol ExampleProtocol
{
    func execute()
}

class Example: ExampleProtocol
{
    init() throws {
        print("one")
    }

    func execute() {
        print("two")
    }
}

let instance: ExampleProtocol

do {
    instance = try Example()
    instance.execute()
} catch (_) {}

In debug this code compiles and runs fine. However, with Swift Compiler Optimization turned on (`-O` with or without whole module optimization), a crashing binary is produced. The binary only outputs "one" and then crashes with a bad access exception.

Additionally, if you leave an empty body for the `init()` method, the compiler crashes.

Attached you'll find an example project to reproduce the issue and the crash log from the compiler.

This bug was found upon compiling a Release version of an App of ours after migrating to Swift 3. This bug was not present in the Swift 2 compiled version.