Replace 'isProvablyNonNull' with existing utility llvm::IsKnownNonNull which handles more cases. Noticed by inspection.

Because of how the IR generation works, this isn't expected to cause an observable difference.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@281979 91177308-0d34-0410-b5e6-96231b3b80d8