RFR: 8342001: GenShen: Factor cases for allocation type into separate methods [v2]

[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:

  Improve comments for new methods

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Changes:
  - all: https://git.openjdk.org/shenandoah/pull/511/files
  - new: https://git.openjdk.org/shenandoah/pull/511/files/0360f2ff..8156aad2

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=shenandoah&pr=511&range=01
 - incr: https://webrevs.openjdk.org/?repo=shenandoah&pr=511&range=00-01

  Stats: 12 lines in 1 file changed: 10 ins; 0 del; 2 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