openmp: Non-rectangular loop support for non-composite worksharing loops and distribute
This implements the fallback mentioned in
https://gcc.gnu.org/pipermail/gcc/2020-June/232874.html
Special cases for triangular loops etc. to follow later, also composite
constructs not supported yet (need to check the passing of temporaries around)
and lastprivate might not give the same answers as serial loop if the last
innermost body iteration isn't the last one for some of the outer loops
(that will need to be solved separately together with rectangular loops that have no
innermost body iterations, but some of the outer loops actually iterate).
Also, simd needs work.