[PM] Simplify how we use the registry by including it only once. Still

[PM] Simplify how we use the registry by including it only once. Still
more verbose than I'd like, but the code really isn't that interesting,
and this still seems vastly simpler than any other solutions I've come
up with. =] Maybe if we get to the 10th IR unit, this will be a problem
in practice.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@225245 91177308-0d34-0410-b5e6-96231b3b80d8