Avoid calling use_size() which could (in theory) be expensive if the global

Avoid calling use_size() which could (in theory) be expensive if the global
has a large number of users.  Instead, just keep track of whether we're
making changes as we do so.

This patch has no functionlity changes.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@16874 91177308-0d34-0410-b5e6-96231b3b80d8