sortperm! allocates due to inference/type stability issues

[LilithHafner]

I don't know exactly why sortperm! allocates here, but I know that it shouldn't:

julia> v = rand(10);

julia> ix = similar(eachindex(v));

julia> @btime sortperm!($ix, $v) setup=(copyto!($ix, eachindex($v))) evals=1;
  939.000 ns (5 allocations: 336 bytes)

julia> @btime sort!($ix) setup=(sortperm!($ix, $v)) evals=1;
  84.000 ns (0 allocations: 0 bytes)

This is not new in 1.9 but it is slightly worse.