[IndVars] Have getInsertPointForUses preserve LCSSA

Summary:
Also add a stricter post-condition for IndVarSimplify.

Fixes PR25578.  Test case by Michael Zolotukhin.

Reviewers: hfinkel, atrick, mzolotukhin

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D15059

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@254977 91177308-0d34-0410-b5e6-96231b3b80d8
4 files changed