[NFCI][Local] Rewrite EliminateDuplicatePHINodes to optionally check hashing invariants

EarlyCSE has a mode to verify the invariant that hash equality equals
key equality, but EliminateDuplicatePHINodes() doesn't.

I've verified that this would have caught the stage2-stage3 mismatches
5ec2b757 revert has fixed,
that were introduced last time in 3e69871a.