LCSSA should be performed on the outermost affected loop while unrolling loop.

During loop-unroll, loop exits from the current loop may end up in in different
outer loop. This requires to re-form LCSSA recursively for one level down from
the outer most loop where loop exits are landed during unroll. This fixes PR18861.

Differential Revision: http://reviews.llvm.org/D2976



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@209796 91177308-0d34-0410-b5e6-96231b3b80d8