[mypyc] feat: optimize away first if-check in for_loop_helper_with_index

[BobTheBuidler]

It is not necessary to enter the gen_condition block on the first iteration, we can safely skip it. This won't be particularly impactful but its a low-hanging fruit.

[JukkaL]

Makes sense. The C compiler may be able to perform a similar optimization in some cases, but not necessarily in every case where we can do it.