Make ScopAnnotator a function-local variable to ensure it is freed at each run

When the ScopAnnotator was a class member variable some of the maps it contains
have not been properly cleared. As a result we had dangling pointers to
llvm::Value(s) which got detected by the AssertingVH we recently added.

No test case as this issue is hard to reproduce reliably as subsequent
optimizations need to delete some of the llvm::Values we still keep in our
lists.

llvm-svn: 249269