cfg_simplify! error

[aviatesk]

MRE:

julia> ir, = Base.code_ircode() do
           while true
               i = @noinline rand(Int)
               if @noinline isodd(i)
                   return i
               end
           end
       end |> only
  1 ─      nothing::Nothing
3 2 ─ %2 = invoke Main.rand(Main.Int::Type{Int64})::Int64
4 │   %3 = invoke Main.isodd(%2::Int64)::Bool
  └──      goto #4 if not %3
5 3 ─      return %2
7 4 ─      goto #1
   => Int64                                                                                                                                                                   

julia> Core.Compiler.cfg_simplify!(ir)
ERROR: AssertionError: label > 0
Stacktrace:
 [1] 
   @ Core.Compiler ./compiler/ssair/ir.jl:1348
 [2] cfg_simplify!(ir::Core.Compiler.IRCode)
   @ Core.Compiler ./compiler/ssair/passes.jl:2276
 [3] top-level scope
   @ REPL[8]:1
Some type information was truncated. Use `show(err)` to see complete types.

This issue particularly blocks #51960, which introduces actual use of cfg_simplify! into the base compilation pipeline. To echo what Keno mentioned in our previous Slack chat:

Keno Fischer
I'd have to look into it, but the thing that strikes me as unusual about this IR is that we loop into the entry block. I wouldn't be surprised if we accidentally used the entry as a magic value somewhere and got confused