[SpillPlacement] Fix a quadratic behavior in spill placement.

The bad behavior happens when we have a function with a long linear chain of
basic blocks, and have a live range spanning most of this chain, but with very
few uses.
Let say we have only 2 uses.
The Hopfield network is only seeded with two active blocks where the uses are,
and each iteration of the outer loop in `RAGreedy::growRegion()` only adds two
new nodes to the network due to the completely linear shape of the CFG.
Meanwhile, `SpillPlacer->iterate()` visits the whole set of discovered nodes,
which adds up to a quadratic algorithm.

This is an historical accident effect from r129188.

When the Hopfield network is expanding, most of the action is happening on the
frontier where new nodes are being added. The internal nodes in the network are
not likely to be flip-flopping much, or they will at least settle down very
quickly. This means that while `SpillPlacer->iterate()` is recomputing all the
nodes in the network, it is probably only the two frontier nodes that are
changing their output.

Instead of recomputing the whole network on each iteration, we can maintain a
SparseSet of nodes that need to be updated:

- `SpillPlacement::activate()` adds the node to the todo list.
- When a node changes value (i.e., `update()` returns true), its neighbors are
  added to the todo list.
- `SpillPlacement::iterate()` only updates the nodes in the list.

The result of Hopfield iterations is not necessarily exact. It should converge
to a local minimum, but there is no guarantee that it will find a global
minimum. It is possible that updating nodes in a different order will cause us
to switch to a different local minimum. In other words, this is not NFC, but
although I saw a few runtime improvements and regressions when I benchmarked
this change, those were side effects and actually the performance change is in
the noise as expected.

Huge thanks to Jakob Stoklund Olesen <stoklund@2pi.dk> for his feedbacks,
guidance and time for the review.

llvm-svn: 263460