Code like:

    val x = if (cond) throw new A else throw new B

Was being transformed to:

    val ifRes = ???
    if (cond) ifRes = throw new A else ifRes = throw new B
    val x = ifRes

by way of the use of `gen.mkZero` which throws `???` if the requested type is `Nothing`

This commit special cases `Nothing` typed expressions in a similar manner to `Unit` type expressions.
The example above is now translated to:

    if (cond) throw new A else throw new B
    val x = throw new IllegalStateException()

Fixes scala#120