[Utils] Clean another duplicated util method.

Summary:
Following the cleanup in D48202, method foldBlockIntoPredecessor has the
same behavior. Replace its uses with MergeBlockIntoPredecessor.
Remove foldBlockIntoPredecessor.

Reviewers: chandlerc, dmgreen

Subscribers: jlebar, javed.absar, zzheng, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D62751

llvm-svn: 362538