[LoopSimplifyCFG] Check predecessors of exits before marking them dead.

LoopSimplifyCFG may process loops that are not in
loop-simplify/canonical form. For loops not in canonical form, exit
blocks may be reachable from non-loop blocks and we cannot consider them
as dead if they only are not reachable from the loop itself.

Unfortunately the smallest test I could come up with requires running
multiple passes:
    -passes='loop-mssa(loop-instsimplify,loop-simplifycfg,simple-loop-unswitch)'

The reason is that loops are canonicalized at the beginning of loop
pipelines, so a later transform has to break canonical form in a way
that breaks LoopSimplifyCFG's dead-exit analysis.

Alternatively we could try to require all loop passes to maintain
canonical form. That in turn would also require additional verification.

Fixes #54023, #49931.

Reviewed By: nikic

Differential Revision: https://reviews.llvm.org/D121925