This change is needed for the next update to ToT LLVM. It can be put
into place now without breaking anything so I am committing it now.
The churn upstream on ilist_node is neccessary to remove undefined
behavior. Rather than updating the different ilist_node patches for the
hacky change required to not use iterators, just use iterators and keep
everything as ilist_nodes. Upstream they want to eventually do this, so
it makes sense for us to just do it now.
Please do not introduce new invocations of
ilist_node::get{Next,Prev}Node() into the tree.
type, though; I had to define a WriteAsOperand function and
add a front() method to SILFunction to get this to work.
Rip out my dominators implementation and replace it with
LLVM's. I'd forgotten that LLVM's was actually generic.
Swift SVN r4717
