Faster extrema on abstract arrays when there is no functions or selected dims

[longemen3000]

for reference, see this discourse thread:
https://discourse.julialang.org/t/perf-improvement-suggestion-for-simple-extrema/34776/2
the key is just writing a simple loop in the case of extrema(x::AbstractArray). the post uses an standard indexing, but i think this can be changed?

[longemen3000]

a prototype

function extrema(x::AbstractArray)
  a = b = first(x)
  @inbounds @simd for i in eachindex(x)
    if x[i] > b 
      b = x[i]
    elseif x[i] < a 
      a = x[i]
    end
  end
  a, b
end

[tkf]

Isn't it more or less a dup of #31442?

[longemen3000]

not technically, but it seems so