[IndVars] Teach replaceCongruentIVs to avoid scrambling induction variables

replaceCongruentIVs analysis is based on ScalarEvolution; this makes
comparing different PHIs and performing the replacement straightforward.
However, it can have some side-effects: it isn't aware whether an
induction variable is in canonical form, so it can perform replacements
which obscure the meaning of the IR.

In test22 in widen-loop-comp.ll, the resulting loop can't be analyzed by
ScalarEvolution at all.

My attempted solution is to restrict the transform: don't try to replace
induction variables using PHI nodes that don't represent simple
induction variables.

I'm not sure if this is the best solution; suggestions welcome.

Differential Revision: https://reviews.llvm.org/D121950