Fix a use-after-free error in GlobalOpt CleanupConstantGlobalUsers

GlobalOpt's CleanupConstantGlobalUsers function uses a worklist array to manage
constant users to be visited. The pointers in this array need to be weak
handles because when we delete a constant array, we may also be holding a
pointer to one of its elements (or an element of one of its elements if we're
dealing with an array of arrays) in the worklist.

Fixes PR17347.

llvm-svn: 197178