[PATCH] Teaching loop simplify to preserve LCSSA

[Andrew Trick]

Ok. But sometimes we need to create new phis after full unrolling. We can have a sequence of blocks initially in the same inner loop. After unrolling, they all end up in different loops and now we need a phi between them. I think we should make this correct first. Yes, LCSSA update is expensive and that is the drawback of doing it.

http://reviews.llvm.org/D2986