[IndVars] Don't break dominance in `eliminateIdentitySCEV`

[IndVars] Don't break dominance in `eliminateIdentitySCEV`

Summary:
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.

Reviewers: atrick, hfinkel

Subscribers: joker.eph, llvm-commits, sanjoy

Differential Revision: http://reviews.llvm.org/D13460

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@249471 91177308-0d34-0410-b5e6-96231b3b80d8