bad PhiNode renumbering of #undef value

[vtjnash]

I accidentally wrote this function, where ct was #undef in state3, but it probably should not have crashed the slot2ssa pass:

julia> function donext(nextstate)
    nextstate === 0x3 && @goto state3
    while @noinline rand(Bool) # any sort of scope block, such as let or while
        ct = @noinline rand(Bool) ? [] : nothing # something type-unstable
        if @noinline rand(Bool)
            return
            @label state3
        end
        Base.donotdelete(ct) # some use of ct
    end
    nothing
end

julia> code_typed(donext, (UInt8,))
ERROR: BoundsError: attempt to access 37-element Vector{Core.SSAValue} at index [-2]
Stacktrace:
  [1] throw_boundserror(A::Vector{Core.SSAValue}, I::Tuple{Int64})
    @ Core.Compiler ./essentials.jl:14
  [2] getindex
    @ ./essentials.jl:904 [inlined]
  [3] renumber_ssa(stmt::Core.SSAValue, ssanums::Vector{Core.SSAValue}, new_ssa::Bool)
    @ Core.Compiler ./compiler/ssair/slot2ssa.jl:55
  [4] #482
    @ ./compiler/ssair/slot2ssa.jl:62 [inlined]
  [5] ssamap(f::Core.Compiler.var"#482#483"{Vector{Core.SSAValue}, Bool}, stmt::Any)
    @ Core.Compiler ./compiler/utilities.jl:392
  [6] renumber_ssa!
    @ ./compiler/ssair/slot2ssa.jl:62 [inlined]
  [7] renumber_ssa!
    @ ./compiler/ssair/slot2ssa.jl:61 [inlined]
  [8] construct_ssa!(ci::Core.CodeInfo, ir::Core.Compiler.IRCode, sv::Core.Compiler.OptimizationState{…}, domtree::Core.Compiler.GenericDomTree{…}, defuses::Vector{…}, 𝕃ₒ::Core.Compiler.PartialsLattice{…})
    @ Core.Compiler ./compiler/ssair/slot2ssa.jl:874
  [9] slot2reg
    @ ./compiler/optimize.jl:1259 [inlined]