RFR: 8308660: C2 compilation hits 'node must be dead' assert [v3]

[Roland Westrelin]

> In `IfNode::fold_compares_helper()`, `adjusted_val` is:
> 
> 
> (SubI (AddI top constant) 0)
> 
> 
> which is then transformed to the `top` node. The code, next, tries to
> destroy the `adjusted_val` node i.e. the `top` node. That results in
> the assert failure. Given We're trying to fold 2 ifs in a dying part
> of the graph, the fix is straightforward: test `adjusted_val` for top
> and bail out from the transformation if that's the case.

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/18305/files
  - new: https://git.openjdk.org/jdk/pull/18305/files/f4703ea7..43a1d337

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=18305&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=18305&range=01-02

  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/18305.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/18305/head:pull/18305

PR: https://git.openjdk.org/jdk/pull/18305