RFR: 8365256: RelocIterator should use indexes instead of pointers

[Johan Sjölen]

Hi,

This PR replaces the `current` and `end` pointers with a `base` pointer alongside a `current` index and a `len`. This allows us to have `-1` as the initial value for current, while retaining `nullptr` as the 'dead' value for `_mutable_data`.

Performance testing shows no difference/performance improvements on DaCapo Linux x64. I don't think that these are actual improvements, but at least there are no clear regressions.

Testing: GHA

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Commit messages:
 - Fix the bug
 - Keep invariant
 - Delete unnecessary assert
 - Change comment
 - Explicitly assign _mutable_data to nullptr
 - Use a base pointer and a -1 index start instead

Changes: https://git.openjdk.org/jdk/pull/26569/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=26569&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8365256
  Stats: 83 lines in 4 files changed: 18 ins; 24 del; 41 mod
  Patch: https://git.openjdk.org/jdk/pull/26569.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/26569/head:pull/26569

PR: https://git.openjdk.org/jdk/pull/26569