RFR: 8365256: RelocIterator should use indexes instead of pointers [v3]

[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

Johan Sjölen has updated the pull request incrementally with one additional commit since the last revision:

  Make constructor private

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/26569/files
  - new: https://git.openjdk.org/jdk/pull/26569/files/e71b4924..f2a4c916

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=26569&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=26569&range=01-02

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