[PATCH] InstCombine: propagate deref via new addDereferenceableAttr

[Ramkumar Ramachandra]

In http://reviews.llvm.org/D7510#120855, @reames wrote:

> p.s. isDerefPtr is on Value, the cast is entirely unnecessary.


Ah, but `->getDereferenceableBytes()` is only defined for an `Argument`; I'm not sure why it is that way though.


http://reviews.llvm.org/D7510

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