[InstSimplify] do not propagate poison from select arm to icmp user

Authored by spatel on Jul 1 2021, 2:30 PM.


[InstSimplify] do not propagate poison from select arm to icmp user

This is the cause of the miscompile in:

The problem has likely existed for some time, but it was made visible with:
5af8bacc94024 ( D104661 )
handleOtherCmpSelSimplifications() assumed it can convert select of
constants to bool logic ops, but that does not work with poison.
We had a very similar construct in InstCombine, so the fix here
mimics the fix there.

The bug is in instsimplify, but I'm not sure how to reproduce it outside of
instcombine. The reason this is visible in instcombine is because we have a
hack (FIXME) to bypass simplification of a select when it has an icmp user:

So we get to an unusual case where we are trying to simplify an instruction
that has an operand that would have already simplified if we had processed
it in normal order.

Differential Revision: https://reviews.llvm.org/D105298