[SimplifyCFG] Improve the precision of `PtrValueMayBeModified`

The result value of `getelementptr inbounds (TY, null, not zero)` is a poison value. We can think of it as undefined behavior.

> Please let me know if there is anything I don't understand correctly.

Reviewed By: nikic, xbolva00

Differential Revision: https://reviews.llvm.org/D144563