Add indvar pass to canonicalization sequence

Running indvar before Polly is useful as this eliminates zexts as they commonly
appear when a 32 bit induction variable (type int) was used on a 64 bit system.
These zexts confuse our delinearization and prevent for example the successful
delinearization of the nussinov kernel in polybench-c-4.1.

This fixes http://llvm.org/PR23426



Suggested-by: Xing Su <xsu.llvm@outlook.com>
llvm-svn: 238643