[SCEV] Use SCEVUnknown(poison) instead of SCEVUnknown(undef).

[fhahn]

Summary

Use poison instead of undef for SCEVUnkown of unreachable values.
This should be in line with the movement to replace undef with poison
when possible.

Suggested in D114650.

[nikic]

llvm/lib/Analysis/ScalarEvolution.cpp

7199

This change is fine, but I don't think the containsUndef -> containsPoison change makes sense. We can still have SCEVUnknown of UndefValue (from other places), and I believe current containsUndef() callers do want to check for those as well.

[fhahn]

llvm/lib/Analysis/ScalarEvolution.cpp

7199

I suppose we would need to check for both undef & poison, which is achieved by just checking for UndefVal (because PoisonValue inherits from UndefValue)?

[fhahn]

Undo changes to containsUndefs.

[nikic]

LGTM

llvm/lib/Analysis/ScalarEvolution.cpp

7199

Yes, exactly.