[GuardWidening] Improve analysis of potential widening into hotter block, try 2

The initial version was reverted because it looped infinitely if the likely successor
isn't properly dominated by the predecessor. In practice it means that we went up the
CFG through backedge and looped infinitely.

I also added some paranoid assertion checks to make sure that every other invariant
holds. I also found a hypothetical situation when we may go past the dominated block
while following the likely successors (it means that in fact the dominated block is
dynamically not reachable from dominating block) and explicitly prohibited this, though
I don't have a motivating test showing that it's a real problem.

https://reviews.llvm.org/D146276