RFR: 8366361: C2 SuperWord: rename VTransformNode::set_req -> init_req, analogue to Node::init_req [v2]

[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
> [See plan overfiew.](https://bugs.openjdk.org/browse/JDK-8340093)
> 
> 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.)

Emanuel Peter has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains five commits:

 - Merge branch 'master' into JDK-8366361-vtn-init_req
 - JDK-8366361
 - For Christian: use phase->intcon instead
 - Update src/hotspot/share/opto/vtransform.hpp
   
   Co-authored-by: Christian Hagedorn <christian.hagedorn at oracle.com>
 - JDK-8366357

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Changes: https://git.openjdk.org/jdk/pull/26991/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=26991&range=01
  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