RFR: 8303755: Clean up around JavacElements.getAllMembers [v6]

[Pavel Rappo]

> Please review this cleanup which consists of commentary changes and code simplifications.

Pavel Rappo has updated the pull request incrementally with two additional commits since the last revision:

 - Drop recomputation improvements
   
   This reverts e18c0444cbe9f325d7122b97531869d2b8fedc56. Both `rank`
   and `supertype` are _cached_ anyway, so the performance is not
   a concern here. As for readability, while it's not really better,
   it stands out from the rest of the file where everything is
   recomputed on demand rather than cached and reused.
   
   However, this adds clarifying parentheses for `rank` in `precedes`.
 - Repair botched merge
   
   6515bf762a0ffb4a8686e07ae37611507cf2b1be

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/12904/files
  - new: https://git.openjdk.org/jdk/pull/12904/files/6cda4b37..fb550f7f

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=12904&range=05
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=12904&range=04-05

  Stats: 6 lines in 2 files changed: 0 ins; 1 del; 5 mod
  Patch: https://git.openjdk.org/jdk/pull/12904.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12904/head:pull/12904

PR: https://git.openjdk.org/jdk/pull/12904