[SCCP] Don't mark edges feasible when resolving undefs

[SCCP] Don't mark edges feasible when resolving undefs

As branch on undef is immediate undefined behavior, there is no need
to mark one of the edges as feasible. We can leave all the edges
non-feasible. In IPSCCP, we can replace the branch with an unreachable
terminator.

Differential Revision: https://reviews.llvm.org/D126962