[LoopSimplifyCFG] Don't delete LCSSA Phis

When removing edges, we also update Phi inputs and may end up removing
a Phi if it has only one input. We should not do it for edges that leave the current
loop because these Phis are LCSSA Phis and need to be preserved.

Thanks @dmgreen	for finding this!

Differential Revision: https://reviews.llvm.org/D54841

llvm-svn: 347484