RFR: 8347718: Unexpected NullPointerException in C2 compiled code due to ReduceAllocationMerges [v2]

[Cesar Soares Lucas]

> Please, review this patch to fix an issue in reducing allocation merges used by Cmp nodes. The attached test case, adapted from the one attached to the JBS issue, can be used to reproduce the issue. The root of the problem was not properly checking the return value of `optimize_ptr_compare` against different Cmp mask predicates.
> 
> I tested this on OSX(ARM64) & Linux(Intel) tier1-4.

Cesar Soares Lucas has updated the pull request incrementally with one additional commit since the last revision:

  Address PR feedback: typo on test & move of igvn.transform

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Changes:
  - all: https://git.openjdk.org/jdk/pull/23312/files
  - new: https://git.openjdk.org/jdk/pull/23312/files/9b0bd41f..42052475

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=23312&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=23312&range=00-01

  Stats: 3 lines in 2 files changed: 0 ins; 0 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/23312.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/23312/head:pull/23312

PR: https://git.openjdk.org/jdk/pull/23312