In a "seeing the forest through the trees" kinda situation, I realized that a

complete rewrite of load-vn will make it a bit faster.  This changes speeds up
the gcse pass (which uses load-vn) from 25.45s to 0.42s on the testcase in
PR209.

I've also verified that this gives the exact same results as the old one.

llvm-svn: 11132