[GlobalMerge] Exit early if only one global is to be merged

To save some compilation time and prevent some unnecessary changes.

Differential Revision: https://reviews.llvm.org/D46640

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@332813 91177308-0d34-0410-b5e6-96231b3b80d8
3 files changed