fix: rewrite initialization, increment and condition for replace_doloop

[gxyd]

The transformation set i = a-c initially, was incorrect because it effectively changes the starting value of the loop variable i.

Fixes #3902

[gxyd]

Seems like the below program with the PR's changes would never end:

program main
    implicit none
    integer(4) :: k
    k = 1
    do while (k <= 4)
        if (k == 3) then
            cycle
        end if
        print *, "variable i =", k
        k = k + 1
    end do
end program main

[Pranavchiku]

I think we did this intentionally, I do not know the reason atm. I commented way to fix #3902 there, basically this is old convention of writing codes, we encountered in SciPy and hence introduced --use-loop-variable-after-loop.

[Pranavchiku]

all these changes will work with --use-loop-variable-after-loop

[gxyd]

That makes sense, I did initially wonder what the purpose of use_loop_variable_after_loop in replace_doloop is.

[gxyd]

I do not know the reason atm

Maybe the possibility of abrupt ending (via exit) or continuation of loop without going to the end (via cycle) might have been the reason.

@certik can you please confirm? If that's the case, I think there is nothing to do here, as we probably need to use --use-loop-variable-after-loop flag

[gxyd]

I'm curious for if above is the reason, isn't it possible to increment the loop variable before appending the cycle & exit statements?