RFR: Do not apply evacuation budgets in non-generational mode

[William Kemper]

CSet selection for non-generational mode was using a value configured on the young generation. In non-generational modes we do not maintain all the attributes of the young generation (instead we maintain the _global_ generation). This caused many cycles in which the maximum CSet was zero (or close to it). This, in turn, caused the collector to run much more frequently (approximately 3x on specjbb) which caused severe performance regression in critical jops.

I'm not sure why the diff algorithm is struggling so much with these changes. I pulled up the `mode()->is_generational()` out of `compute_evacuation_budgets` and `adjust_evacuation_budgets` into the caller and replaced the check with an assert (re-indenting the code in the method).

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Commit messages:
 - Do not apply evacuation budgets in non-generational mode

Changes: https://git.openjdk.org/shenandoah/pull/171/files
 Webrev: https://webrevs.openjdk.org/?repo=shenandoah&pr=171&range=00
  Stats: 738 lines in 2 files changed: 227 ins; 238 del; 273 mod
  Patch: https://git.openjdk.org/shenandoah/pull/171.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah pull/171/head:pull/171

PR: https://git.openjdk.org/shenandoah/pull/171