Factor the PHI and select speculation into a separate rewriter. This

could probably be factored still further to hoist this logic into
a generic helper, but currently I don't have particularly clean ideas
about how to handle that.

This at least allows us to drop custom load rewriting from the
speculation logic, which in turn allows the existing load rewriting
logic to fire. In theory, this could enable vector promotion or other
tricks after speculation occurs, but I've not dug into such issues. This
is primarily just cleaning up the factoring of the code and the
resulting logic.

llvm-svn: 164933