[ConstraintElim] Add A < B if A is an increasing phi for A != B.

This patch adds additional logic to add additional facts for A != B, if
A is a monotonically increasing induction phi. The motivating use case
for this is removing checks when using iterators with hardened libc++,
e.g. https://godbolt.org/z/zhKEP37vG.

The patch pulls in SCEV to detect AddRecs. If possible, the patch adds
the following facts for a AddRec phi PN with StartValue as incoming
value from the loo preheader and B being an upper bound for PN from a
condition in the loop header.

 * (ICMP_UGE, PN, StartValue)
 * (ICMP_ULT, PN, B) [if (ICMP_ULE, StartValue, B)]

The patch also adds an optional precondition to FactOrCheck (the new
DoesHold field) , which can be used to only add a fact if the
precondition holds at the point the fact is added to the constraint
system.

Depends on D151799.

Reviewed By: nikic

Differential Revision: https://reviews.llvm.org/D152730