compiler/ssair: preserve negative NewSSAValue

[KristofferC]

During incremental compaction a forwarded NewSSAValue with a negative id was treated like an SSAValue, producing SSAValue(-n) and tripping renumber_ssa2 bounds checks. Only convert NewSSAValue to SSAValue when the id is positive, so new_new_nodes references remain valid.

🤖 AI fix for #57827. Putting up for consideration.

[Keno]

I think the fix is probably correct.

[Keno]

Could also use an IR-level test.

[KristofferC]

julia> g = (acc, x) -> ifelse(x > acc[1], (x,), (acc[1],))
#2 (generic function with 1 method)

julia> rf = Base.BottomRF(g)
Base.BottomRF{var"#2#3"}(var"#2#3"())

julia> optimize_until = "CC: COMPACT_2";

# ok
julia> Base.code_ircode(Base._foldl_impl, (typeof(rf), Tuple{Float64}, Vector{Int64}); optimize_until)
1-element Vector{Any}:
54 1 ── %1   = intrinsic Base.sub_int(1, 1)::Int64  iterate
   │    %2   = intrinsic Base.bitcast(Base.UInt, %1)::UInt64

...

julia> optimize_until = "CC: SROA";

# bad
julia> Base.code_ircode(Base._foldl_impl, (typeof(rf), Tuple{Float64}, Vector{Int64}); optimize_until)
ERROR: BoundsError: attempt to access 183-element Vector{Int64} at index [-13]
Stacktrace:
  [1] throw_boundserror(A::Vector{Int64}, I::Tuple{Int64})
    @ Base ./essentials.jl:13
  [2] checkbounds
    @ ./essentials.jl:385 [inlined]
  [3] getindex
    @ ./essentials.jl:964 [inlined]
  [4] renumber_ssa2(val::Core.SSAValue, ssanums::Vector{…}, used_ssas::Vector{…}, new_new_used_ssas::Vector{…}, do_rename_ssa::Bool, mark_refined!::Compiler.Refiner)

[KristofferC]

I didn't manage to produce a pure IR test, but I managed to isolate it into locally defined functions without Base dependencie,s which might be ok for now.