RFR: [lw5] fixing bugs on experimental code to generate Q-descriptors, plus code refactoring

[Vicente Romero]

This fix is fixing some bugs introduced by the experimental code to generate Q-descriptors. Some changes are also reusing some of the experimental code for the benefit of LW5.

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Commit messages:
 - [lw5] fixing bugs introduced by experimental code

Changes: https://git.openjdk.org/valhalla/pull/900/files
 Webrev: https://webrevs.openjdk.org/?repo=valhalla&pr=900&range=00
  Stats: 39 lines in 8 files changed: 14 ins; 3 del; 22 mod
  Patch: https://git.openjdk.org/valhalla/pull/900.diff
  Fetch: git fetch https://git.openjdk.org/valhalla.git pull/900/head:pull/900

PR: https://git.openjdk.org/valhalla/pull/900