[lto] Remove alias pairs for removed cgraph nodes

[Rafael Espindola]

> Jan, if you can suggest a better alternative to this, I'd gladly
> implement it.

One thing that we could do in change when we write IL to disk, but
given that we want to run IPA when combining the IL files and that GCC
passes have nasty inter dependencies, this would be a lot of work.

Cheers,
-- 
Rafael Avila de Espindola

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