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

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.

Reviewed By: nikic

Differential Revision: https://reviews.llvm.org/D128586