This adds a very simplistic reduction of {CheckPoint} nodes via the new
{CheckpointElimination}, eliminating redundant check points that appear
in an effect chain of operations that are all {kNoWrite}. Such a chain
allows an arbitrary check point to be chosen. The current approach will
end up choosing the first one for all deopts in the chain.
R=bmeurer@chromium.org
TEST=unittests/CheckpointEliminationTest.CheckPointChain
BUG=v8:5021
Review-Url: https://codereview.chromium.org/2022913003
Cr-Commit-Position: refs/heads/master@{#36634}
