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

[William Kemper]

Clean backport.

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

Commit messages:
 - Backport 81b631fbc7fc7078786b11c7a85971735ce93b86

Changes: https://git.openjdk.org/shenandoah-jdk21u/pull/142/files
  Webrev: https://webrevs.openjdk.org/?repo=shenandoah-jdk21u&pr=142&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8342001
  Stats: 293 lines in 2 files changed: 140 ins; 105 del; 48 mod
  Patch: https://git.openjdk.org/shenandoah-jdk21u/pull/142.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah-jdk21u.git pull/142/head:pull/142

PR: https://git.openjdk.org/shenandoah-jdk21u/pull/142