This commit fixes the first problem that was found while digging into
JuliaLang/julia#53613. It turns out that the post-domtree constructed
from regular `IRCode` doesn't work for visiting conditional successors
for post-opt analysis in cases like:
```julia
julia> let code = Any[
               # block 1
               GotoIfNot(Argument(2), 3),
               # block 2
               ReturnNode(Argument(3)),
               # block 3 (we should visit this block)
               Expr(:call, throw, "potential throw"),
               ReturnNode(), # unreachable
           ]
           ir = make_ircode(code; slottypes=Any[Any,Bool,Bool])
           visited = BitSet()
           @test !Core.Compiler.visit_conditional_successors(CC.LazyPostDomtree(ir), ir, #=bb=#1) do succ::Int
               push!(visited, succ)
               return false
           end
           @test 2 ∉ visited
           @test 3 ∈ visited
       end
Test Failed at REPL[14]:16
  Expression: 2 ∉ visited
   Evaluated: 2 ∉ BitSet([2])
```

This might mean that we need to fix on the `postdominates` end, but for
now, this commit tries to get around it by using the augmented post
domtree in `visit_conditional_successors`. Since the augmented post
domtree is enforced to have a single return, we can keep using the
current `postdominates` to fix the issue.

However, this commit isn't enough to fix the NeuralNetworkReachability
segfault as reported in #53613, and we need to tackle the second issue
reported there too
(https://github.com/JuliaLang/julia/issues/53613#issuecomment-1983243419).