[RuntimeUnrolling] Update DomTree correctly when exit blocks have successors
When we runtime unroll with multiple exit blocks, we also need to update the
immediate dominators of the immediate successors of the exit blocks.
Reviewers: reames, mkuper, mzolotukhin, apilipenko
Reviewed by: mzolotukhin
Differential Revision: https://reviews.llvm.org/D35304