Avoid argmin, argmax evaluation.

* Test on argmin((a + 5) * 2) and array of 100.000.000 elts
* old implem: 803 ms
* new implem: 582 ms
* CPython : 776 ms
* Same on 2D array:
* old implem: 790 ms
* new implem: 536 ms
* CPython : 783 ms