RFR: 8224980: FLAG_SET_ERGO silently ignores invalid values

[Ioi Lam]

Clarify the `FLAG_SET_ERGO` API - the caller must ensure that a valid value is passed. Added an assert to check this.

Fixed two issues found by the assert:

- `NonNMethodCodeHeapSize`
- `XXXThreshold` and `XXXNotifyFreqLog` flags in `CompilerConfig::set_compilation_policy_flags()`

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Commit messages:
 - 8224980: FLAG_SET_ERGO silently ignores invalid values

Changes: https://git.openjdk.org/jdk/pull/12549/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=12549&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8224980
  Stats: 41 lines in 5 files changed: 22 ins; 1 del; 18 mod
  Patch: https://git.openjdk.org/jdk/pull/12549.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12549/head:pull/12549

PR: https://git.openjdk.org/jdk/pull/12549