Further refine the accuracy of IGV's approximated schedules w.r.t. C2.

Possible refinements include:
- schedule projections and Parm nodes in the same block as their parents,
- ensure that all Region nodes are marked by C2 with the "is_block_start" property,
- revisit IGV's definition of "CFG node" to exclude machine nodes such as "prefetchAlloc" and "rep_stos", and

Done:
- ensure definers are scheduled locally before their users (!), see attached graph wrong-lcm.xml.

The attached graphs illustrate all these inaccuracies.