RFR: 8263577: C2: reachable nodes shouldn't have dead uses at the end of optimizations [v3]

[Roland Westrelin]

> It's not uncommon for c2 to leave dead uses hanging from reachable
> nodes. It's usually harmless but can cause confusion (such as in
> valhalla JDK-8262289, not reproducible with non valhalla code AFAICT).
> 
> Most of the time, these are caused by calls to set_req() during igvn
> that should be set_req_X() calls. I added a verification pass at the
> end of optimization and fixed all the ones I found.

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

  Nils' review

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

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/3012/files
  - new: https://git.openjdk.java.net/jdk/pull/3012/files/f214361e..44a1e43f

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=3012&range=02
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=3012&range=01-02

  Stats: 4 lines in 1 file changed: 0 ins; 0 del; 4 mod
  Patch: https://git.openjdk.java.net/jdk/pull/3012.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/3012/head:pull/3012

PR: https://git.openjdk.java.net/jdk/pull/3012