SCEV should forget all loops containing a deleted block.

Summary:
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

llvm-svn: 339363