[InstCombine] avoid crashing on attribute propagation

[InstCombine] avoid crashing on attribute propagation

In https://llvm.org/PR48810 , we are crashing while trying to
propagate attributes from mempcpy (returns void*) to memcpy
(returns nothing - void).

We can avoid the crash by removing known incompatible
attributes for the void return type.

I'm not sure if this goes far enough (should we just drop all
attributes since this isn't the same function?). We also need
to audit other transforms in LibCallSimplifier to make sure
there are no other cases that have the same problem.

Differential Revision: https://reviews.llvm.org/D95088