[SimpleLoopUnswitch] After unswitch delete dead blocks in parent loops

Summary:
Assert from PR38737 happens on the dead block inside the parent loop
after unswitching nontrivial switch in the inner loop.

deleteDeadBlocksFromLoop now takes extra care to detect/remove dead
blocks in all the parent loops in addition to the blocks from original
loop being unswitched.

Reviewers: asbirlea, chandlerc

Reviewed By: asbirlea

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D51415

llvm-svn: 340955