Update vectorization debug info unittest.

Summary:
The change will test the change in r286159.
The idea behind the change: Make the dbg location different between loop header and preheader/exit. Originally, dbg location 21 exists in 3 BBs: preheader, header, critical edge (exit). Update the debug location of inside the loop header from !21 to !22 so that it will reflect the correct location.

Reviewers: probinson

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D26428

llvm-svn: 286403