[PM] Fix a silly bug in my recent update to the CG update logic.

[PM] Fix a silly bug in my recent update to the CG update logic.

I used the wrong variable to update. This was even covered by a unittest
I wrote, and the comments for the unittest were correct (if confusing)
but the test itself just matched the buggy behavior. =[

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@307764 91177308-0d34-0410-b5e6-96231b3b80d8