After r249211, getSCEV(X) == getSCEV(Y) does not guarantee that X and
Y are related in the dominator tree, even if X is an operand to Y (I've
included a toy example in comments, and a real example as a test case).
This commit changes SimplifyIndVar to require a DominatorTree. I
don't think this is a problem because ScalarEvolution requires it
anyway.
Fixes PR25051.
Depends on D13459.