RFR: Fixes for running in non-generational modes

[William Kemper]

A couple of spots that need to be guarded with a check for generational mode.

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Commit messages:
 - Fixes for running in non-generational modes

Changes: https://git.openjdk.java.net/shenandoah/pull/114/files
 Webrev: https://webrevs.openjdk.java.net/?repo=shenandoah&pr=114&range=00
  Stats: 7 lines in 2 files changed: 5 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/shenandoah/pull/114.diff
  Fetch: git fetch https://git.openjdk.java.net/shenandoah pull/114/head:pull/114

PR: https://git.openjdk.java.net/shenandoah/pull/114