RFR: 8366361: C2 SuperWord: rename VTransformNode::set_req -> init_req, analogue to Node::init_req

[Emanuel Peter]

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.

FYI: this PR is dependent on https://github.com/openjdk/jdk/pull/26987. I'll rebase once that one is integrated. We can still already review, so that the process is a little faster later on. (I have more small changes coming, but separating makes them more reviewable.)

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Depends on: https://git.openjdk.org/jdk/pull/26987

Commit messages:
 - JDK-8366361

Changes: https://git.openjdk.org/jdk/pull/26991/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=26991&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8366361
  Stats: 26 lines in 3 files changed: 0 ins; 0 del; 26 mod
  Patch: https://git.openjdk.org/jdk/pull/26991.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/26991/head:pull/26991

PR: https://git.openjdk.org/jdk/pull/26991