Fix findminmax in the presence of NaN.

[maleadt]

Otherwise:

julia> while true
           x = rand(Float32, 1000)
           Is = rand(1:1000, 2)
           x[Is] .= NaN
           if !isequal(findmin(x), findmin(CuArray(x)))
               @show Is
               @show findmin(x)
               @show findmin(CuArray(x))
               break
           end
       end
Is = [734, 990]
findmin(x) = (NaN32, 734)
findmin(CuArray(x)) = (NaN32, 990)