RFR: 8376604: C2: EA should assert is_oop_field for AddP with oop outs

[Aleksey Shipilev]

Split out of [JDK-8376514](https://bugs.openjdk.org/browse/JDK-8376514) in Valhalla: I think we need to verify more thoroughly that if we reply is_oop_field = false for AddP, then there are no nodes that we feed into oops. We handle it pretty well in various branches in the method already, and we "just" need to check it at the end. Valhalla catches fire on that post-condition check, tracked in [JDK-8376514](https://bugs.openjdk.org/browse/JDK-8376514).

Cleans up the code a bit as well.

Additional testing:
 - [x] Linux x86_64 server fastdebug, `applications/ctw/modules`
 - [x] Linux x86_64 server fastdebug, `hotspot_compiler`

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Commit messages:
 - Adjust assert message
 - More verification

Changes: https://git.openjdk.org/jdk/pull/29468/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=29468&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8376604
  Stats: 20 lines in 2 files changed: 10 ins; 6 del; 4 mod
  Patch: https://git.openjdk.org/jdk/pull/29468.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/29468/head:pull/29468

PR: https://git.openjdk.org/jdk/pull/29468