Enable global address legalization, fixing a todo and allowing the removal

Enable global address legalization, fixing a todo and allowing the removal
of some code.  This exposes the implicit load from the stubs to the DAG, allowing
them to be optimized by the dag combiner.  It also moves darwin specific stuff
out of the isel into the legalizer, and allows more to be moved to the .td file.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@24397 91177308-0d34-0410-b5e6-96231b3b80d8