[LoopInterchange] Skip zext instructions when looking for induction var.

Summary:
SimplifyIndVar may introduce zext instructions to widen arguments of the
loop exit check. They should not prevent us from splitting the loop at
the induction variable, but maybe the check should be more conservative,
e.g. making sure it only extends arguments used by a comparison?

Reviewers: karthikthecool, mcrosier, mzolotukhin

Reviewed By: mcrosier

Subscribers: mzolotukhin, llvm-commits

Differential Revision: https://reviews.llvm.org/D34879

llvm-svn: 311783