[patch] Clarify and verify what llvm.used is

[Rafael Espíndola]

When reviewing my previous patch to GlobalOpt Duncan raised some
questions about the semantics of llvm.used. We should probably clarify
the documentation and have the verifier check it before anything else.

The attached patch makes it explicit that aliases can be put in
llvm.used and changes the verifier to check llvm.used only has
functions, variables and aliases.

Duncan, are you OK with this definition? Is the patch OK?

Cheers,
Rafael
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