[LoopSimplifyCFG] Change logic of dead loops removal to avoid hitting asserts

The function `LI.erase` has some invariants that need to be preserved when it
tries to remove a loop which is not the top-level loop. In particular, it
requires loop's preheader to be strictly in loop's parent. Our current logic
of deletion of dead blocks may erase the information about preheader before we
handle the loop, and therefore we may hit this assertion.

This patch changes the logic of loop deletion: we make them top-level loops
before we actually erase them. This allows us to trigger the simple branch of
`erase` logic which just detatches blocks from the loop and does not try to do
some complex stuff that need this invariant.

Thanks to @uabelho for reporting this!

Differential Revision: https://reviews.llvm.org/D57221
Reviewed By: fedor.sergeev

llvm-svn: 353813