Hoist the contents of Loops in depth first order in the dominator tree,

rather than in random order.  This causes LICM to be DRAMATICALLY more
effective. For example, on bzip2.c, it is able to hoist 302 loads and
2380 total instructions, as opposed to 44/338 before.  This  obviously
makes other transformations much more powerful as well!

llvm-svn: 3978