Change NonLocalDeps to be a densemap of pointers to densemap

instead of containing them by value.  This increases the density
(!) of NonLocalDeps as well as making the reallocation case 
faster.  This speeds up gvn on 403.gcc by 2% and makes room for
future improvements.

I'm not super thrilled with having to explicitly manage the new/delete
of the map, but it is necesary for the next change.

llvm-svn: 60271