RFR: JDK-8315478: GenShen: Tolerate round-off errors in preselected promotion budget

[Kelvin Nilsen]

An assert failure identified an error in previous implementation.  See JBS ticket for description of issue and reproducer.

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Commit messages:
 - Remove instrumentation in place for debugging
 - Add instrumentation to confirm root cause of problem

Changes: https://git.openjdk.org/shenandoah/pull/317/files
 Webrev: https://webrevs.openjdk.org/?repo=shenandoah&pr=317&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8315478
  Stats: 20 lines in 1 file changed: 19 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/shenandoah/pull/317.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah.git pull/317/head:pull/317

PR: https://git.openjdk.org/shenandoah/pull/317