RFR: 8335922: Incorrect @Stable usage of LambdaForm$Name.index [v3]

[Chen Liang]

> The `@Stable` on the `index` field is incorrect, as stable only avoids inlining `0`. Solution is to use bit flip on the actual index (and rename the field to `flippedIndex`), so we use -1 to -256 (mapping to 0 to 255) and 0 the default value is used as an unset indicator.

Chen Liang has updated the pull request incrementally with one additional commit since the last revision:

  Encapsulate offsetIndex, share computation result

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Changes:
  - all: https://git.openjdk.org/jdk/pull/20178/files
  - new: https://git.openjdk.org/jdk/pull/20178/files/0743c5f9..db805834

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=20178&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=20178&range=01-02

  Stats: 4 lines in 1 file changed: 1 ins; 0 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/20178.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/20178/head:pull/20178

PR: https://git.openjdk.org/jdk/pull/20178