Add an initial hack at legalizing GlobalAddress into the appropriate nodes

Add an initial hack at legalizing GlobalAddress into the appropriate nodes
on Darwin to remove smarts from the isel.  This is currently disabled by
default (uncomment setOperationAction(ISD::GlobalAddress to enable it).
tblgen needs to become smarter about tglobaladdr nodes and bigger patterns
needed to be added to the .td file.  However, we can currently emit stuff like
this:  :)

        li r2, lo16(L_x$non_lazy_ptr)
        lis r3, ha16(L_x$non_lazy_ptr)
        lwzx r2, r3, r2

The obvious improvements will follow.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@24390 91177308-0d34-0410-b5e6-96231b3b80d8