RFR: 8373524: C2: no reachable node should have no use [v3]

[Roland Westrelin]

> The failure occurs because `PhiNode::Ideal` uses `set_req` to update
> an input of a `Phi`. That causes the previous input to be disconnected
> but because of the use of `set_req`, the previous input that has no
> use is not enqueued for `igvn` to be reclaimed. The fix is to use
> `set_req_X` instead. I replaced uses of `set_req` with `set_req_X` in
> `PhiNode::Ideal` where I thought it made sense.

Roland Westrelin has updated the pull request incrementally with one additional commit since the last revision:

  review

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/28841/files
  - new: https://git.openjdk.org/jdk/pull/28841/files/9f51aa5b..ac789382

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=28841&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=28841&range=01-02

  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/28841.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/28841/head:pull/28841

PR: https://git.openjdk.org/jdk/pull/28841