[LoopUnroll] Avoid unnecessary DT recomputation.

Summary:
When we completely unroll a loop, it's pretty easy to update DT in-place and
thus avoid rebuilding it. DT recalculation is one of the most time-consuming
tasks in loop-unroll, so avoiding it at least in case of full unroll should be
beneficial.

On some extreme (but still real-world) tests this patch improves compile time by
~2x.

Reviewers: escha, jmolloy, hfinkel, sanjoy, chandlerc

Subscribers: joker.eph, sanjoy, llvm-commits

Differential Revision: http://reviews.llvm.org/D17473

llvm-svn: 261595