[LoopSimplifyCFG] Pay respect to LCSSA when removing dead blocks

Utility function that we use for blocks deletion always unconditionally removes
one-input Phis. In LoopSimplifyCFG, it can lead to breach of LCSSA form.
This patch alters this function to keep them if needed.

Differential Revision: https://reviews.llvm.org/D57231
Reviewed By: fedor.sergeev


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@353803 91177308-0d34-0410-b5e6-96231b3b80d8