[RuntimeUnrolling] Update DomTree correctly when exit blocks have successors

[RuntimeUnrolling] Update DomTree correctly when exit blocks have successors

Summary:
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

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D35304

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@307909 91177308-0d34-0410-b5e6-96231b3b80d8