RFR: 8373343: C2: verify AddP base input only set for heap addresses

[Roland Westrelin]

The base input of `AddP` is expected to only be set for heap accesses
but I noticed some inconsistencies so I added an assert in the `AddP`
constructor and fixed issues that it caught. AFAFICT, the
inconsistencies shouldn't create issues.

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Commit messages:
 - more
 - more
 - more
 - undo
 - exps

Changes: https://git.openjdk.org/jdk/pull/28769/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=28769&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8373343
  Stats: 80 lines in 14 files changed: 21 ins; 7 del; 52 mod
  Patch: https://git.openjdk.org/jdk/pull/28769.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/28769/head:pull/28769

PR: https://git.openjdk.org/jdk/pull/28769