RFR: 8342239: GenShen: Revert changes in adaptive heuristic to avoid overflow on 32 bit

[William Kemper]

One of our changes to increase precision comes with the risk of overflow on 32 bit platforms. We're going to revert it for safety's sake.

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

Commit messages:
 - Avoid overflow on 32bit platforms

Changes: https://git.openjdk.org/shenandoah/pull/513/files
  Webrev: https://webrevs.openjdk.org/?repo=shenandoah&pr=513&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8342239
  Stats: 2 lines in 1 file changed: 0 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/shenandoah/pull/513.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah.git pull/513/head:pull/513

PR: https://git.openjdk.org/shenandoah/pull/513