[PM/Unswitch] Fix a bug in the domtree update logic for the new unswitch

pass.

The original logic only considered direct successors of the hoisted
domtree nodes, but that isn't really enough. If there are other basic
blocks that are completely within the subtree, their successors could
just as easily be impacted by the hoisting.

The more I think about it, the more I think the correct update here is
to hoist every block on the dominance frontier which has an idom in the
chain we hoist across. However, this is subtle enough that I'd
definitely appreciate some more eyes on it.

Sadly, if this is the correct algorithm, it requires computing a (highly
localized) dominance frontier. I've done this in the simplest (IE, least
code) way I could come up with, but that may be too naive. Suggestions
welcome here, dominance update algorithms are not an area I've studied
much, so I don't have strong opinions.

In good news, with this patch, turning on simple unswitch passes the
LLVM test suite for me with asserts enabled.

Differential Revision: https://reviews.llvm.org/D32740

llvm-svn: 303843