Omit the ret slot alloca when it's not needed

[pcwalton]

Clang does not always emit an alloca for the ret slot; we likewise shouldn't.

[Aatch]

@pcwalton could you elaborate? When does clang not do it? (Just trying to make contributor's lives easier)

[dotdash]

I have implemented what clang does in dotdash@dda1467 but need to clean the thing up.