Make DT a transitive dependency of LI.

Summary:
LoopInfoWrapperPass::verify uses DT, which means DT must be alive
even if it has no direct users.

Fixes a crash in expensive checks mode.

Reviewers: pcc, leonardchan

Subscribers: hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D64896

llvm-svn: 366388