[LoopSimplifyCFG] Form LCSSA when a parent loop becomes a sibling

During the transforms in LoopSimplifyCFG, when we remove a dead exiting edge, the
parent loop may stop being reachable from the child loop, and therefore they become
siblings. If the former child loop had uses of some values from its former parent loop,
now such uses will require LCSSA Phis, even if they weren't needed before. So we must
form LCSSA for all loops that stopped being ancestors of the current loop in this case.

Differential Revision: https://reviews.llvm.org/D56144
Reviewed By: fedor.sergeev

llvm-svn: 351434