This change is requred for llvm 4.0. GHC doesn't use that version yet,
but this change is just as valid for versions eariler than 4.0.

Two changes needed:

* Previously, GHC defined a `topN` node in the TBAA heiarchy and some IR
  instructions referenced that node. With LLVM 4.0 the root node can no
  longer be referenced by IR instructions, so we introduce a new element
  `rootN` and make `topN` a child of that.

* Previously the root TBAA node was rendered as "!0 = !{!"root", null}".
  With LLVM 4.0 that needs to be "!0 = !{!"root"}" which is also
  accepted by earlier versions.

Test Plan: Build with quick-llvm BuildFlavor and run tests

Reviewers: bgamari, drbo, austin, angerman, michalt, DemiMarie

Reviewed By: DemiMarie

Subscribers: mpickering, DemiMarie, thomie

Differential Revision: https://phabricator.haskell.org/D2975