[PATCH] Simplify n-ary adds by reassociation

[Sanjoy Das]

I'm okay with this living as a separate pass.  I think this pass exposes clear, orthogonal functionality and can also be made more efficient by the virtue of working on an entire function at once.

Minor comments inline, all of which are okay to fix either pre or post commit, as you see fit.


http://reviews.llvm.org/D8950

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