[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

llvm-svn: 316582