[Local] Fix a bug in the domtree update logic for MergeBasicBlockIntoOnlyPred.

Summary: For some irreducible CFG the domtree nodes might be dead, do not update domtree for dead nodes.

Reviewers: kuhar, dberlin, hfinkel

Reviewed By: kuhar

Subscribers: llvm-commits, mcrosier

Differential Revision: https://reviews.llvm.org/D38960

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@316582 91177308-0d34-0410-b5e6-96231b3b80d8