GlobalOpt: fix an issue where CXAAtExitFn points to a deleted function.

CXAAtExitFn was set outside a loop and before optimizations where functions
can be deleted. This patch will set CXAAtExitFn inside the loop and after
optimizations.

Seg fault when running LTO because of accesses to a deleted function.
rdar://problem/13838828

llvm-svn: 181838