SCEV should forget all loops containing a deleted block.
LoopSimplifyCFG should update ScEv for all loops after a block is deleted.
If the deleted block "Succ" is part of L, then it is part of all parent loops, so forget topmost loop.
Reviewers: greened, mkazantsev, sanjoy
Subscribers: jlebar, javed.absar, uabelho, llvm-commits
Differential Revision: https://reviews.llvm.org/D50422