Simplify blockaddress usage before giving up in MergeBlockIntoPredecessor

Summary:
Simplify blockaddress usage before giving up in MergeBlockIntoPredecessor

This is a missing small optimization in MergeBlockIntoPredecessor.

This helps with one simplifycfg test which expects this case to be handled.

Reviewers: davide, spatel, brzycki, asbirlea

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D48284

llvm-svn: 334992