[PATCH] [SDAG] Introduce a combined set to the DAG combiner which tracks nodes which have successfully round-tripped through the combine phase, and use this to ensure all operands to DAG nodes are visited by the combiner, even if they are only added...

[Chandler Carruth]

FYI, discussed this with some folks on IRC too and they seem happy with it.

However, I want to land http://reviews.llvm.org/D4654 (or something similar) first as that will remove the regressions w.r.t. sext from this change.

http://reviews.llvm.org/D4638