RFR: 8360206: Refactor ReferenceProcessor::balance_queues

[Albert Mingkun Yang]

Simple refactoring to clean up the inner-loop conditions and control flow. Due to the structural change, the diff is slightly large and may be harder to read than the resulting code. The new version simplifies the logic by using a single `remaining_to_move` counter in the inner loop to track remaining work, and introduces early returns to reduce indentation and improve clarity.

Test: tier1-3

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

Commit messages:
 - ref-refactor

Changes: https://git.openjdk.org/jdk/pull/25930/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=25930&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8360206
  Stats: 71 lines in 1 file changed: 19 ins; 22 del; 30 mod
  Patch: https://git.openjdk.org/jdk/pull/25930.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/25930/head:pull/25930

PR: https://git.openjdk.org/jdk/pull/25930