To avoid unnecessary forward references, the reader doesn't process
initializers of `GlobalValue`s until after the constant pool has been
processed, and then in reverse order. Model this when predicting
use-list order. This gets two more Bitcode tests passing with
`llvm-uselistorder`.
Part of PR5680.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@214242 91177308-0d34-0410-b5e6-96231b3b80d8