RFR: 8308660: C2 compilation hits 'node must be dead' assert [v2]

[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:

  fix test

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/18305/files
  - new: https://git.openjdk.org/jdk/pull/18305/files/32a52ecd..f4703ea7

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=18305&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=18305&range=00-01

  Stats: 5 lines in 1 file changed: 3 ins; 0 del; 2 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