RFR: 8342001: GenShen: Factor cases for allocation type into separate methods [v4]

[William Kemper]

> Prune switch statement which has grown unwieldy.

William Kemper has updated the pull request incrementally with one additional commit since the last revision:

  Add comment explaining why collector perfers to steal regions from the right

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Changes:
  - all: https://git.openjdk.org/shenandoah/pull/511/files
  - new: https://git.openjdk.org/shenandoah/pull/511/files/86d75325..8ed69adc

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=shenandoah&pr=511&range=03
 - incr: https://webrevs.openjdk.org/?repo=shenandoah&pr=511&range=02-03

  Stats: 2 lines in 1 file changed: 2 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/shenandoah/pull/511.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah.git pull/511/head:pull/511

PR: https://git.openjdk.org/shenandoah/pull/511