[Flang][OpenMP] Separate creation of work-sharing and SIMD loops, NFC (#77757)

These two constructs were both handled in `genOMP` for loop constructs.
There is some shared code between the two, but there are also enough
differences to separate these two cases into individual functions.

The shared code converting loop bounds and steps has been extracted
into a separate function.

Recursive lowering [1/5]