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

[William Kemper]

Prune switch statement which has grown unwieldy.

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Commit messages:
 - Reorder declarations to match order of definitions
 - Factor allocation type cases into separate methods

Changes: https://git.openjdk.org/shenandoah/pull/511/files
  Webrev: https://webrevs.openjdk.org/?repo=shenandoah&pr=511&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8342001
  Stats: 279 lines in 2 files changed: 126 ins; 105 del; 48 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