[GlobalOpt] Pass DTU to removeUnreachableBlocks instead of recomputing.

removeUnreachableBlocks knows how to preserve the DomTree, so make use
of it instead of re-computing the DT.

Reviewers: davide, kuhar, brzycki

Reviewed By: davide, kuhar

Differential Revision: https://reviews.llvm.org/D68298

llvm-svn: 373430