RFR: 8342001: GenShen: Factor cases for allocation type into separate methods [v3]

[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:

  Review feedback

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Changes:
  - all: https://git.openjdk.org/shenandoah/pull/511/files
  - new: https://git.openjdk.org/shenandoah/pull/511/files/8156aad2..86d75325

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=shenandoah&pr=511&range=02
 - incr: https://webrevs.openjdk.org/?repo=shenandoah&pr=511&range=01-02

  Stats: 10 lines in 2 files changed: 2 ins; 1 del; 7 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