[PATCH] Teaching loop simplify to preserve LCSSA

[Dinesh Dwivedi]

  gentle ping

  To my understanding, this change will not leave many unnecessory phi nodes around. Loop was in LCSSA form and to keep it in that way, we can not discard phi node even if there is only one incomming node and as commented in many places in code, keeping extra phi nodes will not trouble us in further analysis and transform. Ultimately they will be removed for the IR.

  I have tested this with few test cases and resulting IR at the end of pass is same as with the old code where we were using formLCSSARecursively() call.

http://llvm-reviews.chandlerc.com/D2986