I'm working on cost-modeling, and am integrating some smaller changes from this proof-of-concept PR:
https://github.com/openjdk/jdk/pull/20964

This is a pure refactoring - no change in behaviour. I'm presenting it like this because it will make reviews easier.

The current implementation of VTransformNode::set_req has init_req semantics, it verifies that the corresponding input is still nullptr. We should thus rename it. It will also free up the "set_req" name for later use in VTransform optimizations, where we want to modify the graph.

See VTransformReductionVectorNode::optimize_move_non_strict_order_reductions_out_of_loop in the proof-of-concept PR.