* Grab Bag of minor cleanups to LowerParallelTasks

Basically OCD code stuff I noted down when debugging the issues, this restructures the inner loop to avoid calling a local function that has non-obvious side effects (setting just the right slot in the closure args), as well as consolidating via helper functions, hoisting common stuff used in both paths, using std::move where seemingly appropriate, adding some (hopefully correct) comments about arg expectations, and other things that aren't likely to really move the needle in terms of Halide compile speed, but (hopefully) make the code a little bit more understandable after some time away.

(There was a todo about "find a better place for generate_closure_ir()"; this PR eliminates it entirely, just inlining it into the caller, which I think is reasonable given thhe number of assumptions the caller has to make in the first place...)

* Update LowerParallelTasks.cpp