[IRTests] Verify PDT instead of DT

Summary: Fix two typos which result in verifying wrong data structures (DT) instead of PDT in DominatorTreeBatchUpdatesTest.

Reviewers: davide, kuhar, grosser, dberlin

Reviewed By: davide, kuhar, dberlin

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D46696

llvm-svn: 332086