Changes

[NFC] [LoopPeel] Change the way DT is updated for loop exits

When peeling a loop, we assume that the latch has a `br` terminator and
that all loop exits are either terminated with an `unreachable` or have
a terminating deoptimize call. So when we peel off the 1st iteration, we
change the IDom of all loop exits to the peeled copy of
`NCD(IDom(Exit), Latch)`. This works now, but if we add logic to support
loops with exits that are followed by a block with an `unreachable` or a
terminating deoptimize call, changing the exit's idom wouldn't be enough
and DT would be broken.

For example, let `Exit1` and `Exit2` are loop exits, and each of them
unconditionally branches to the same `unreachable` terminated block. So
neither of the exits dominates this unreachable block. If we change the
IDoms of the exits to some peeled loop block, we don't update the
dominators of the unreachable block. Currently we just don't get to the
peeling logic, saying that we can't peel such loops.

With this NFC we just insert edges from cloned exiting blocks to their
exits after peeling each iteration (we accumulate the insertion updates
and then after peeling apply the updates to DT).

This patch was a part of D110922.

Patch by Dmitry Makogon!

Differential Revision: https://reviews.llvm.org/D111611
Reviewed By: mkazantsev

[JITLink] Add comments, rename types for visitExistingEdges utility.

The "Fixers" name was a hangover from an earlier draft of the patch. "Visitors"
fits the function name(s).