RFR: Simplify G1CodeBlobClosure

[Stefan Karlsson]

There's no need to introduce a new _keepalive_is_strong property in the G1CodeBlobClosure. The existing _strong is set to true when the evacuation participates in the marking of the old gen, which is when we need to mark the nmethods as "maybe on continuation". The name _strong in this context is not great and it doesn't convey the actual meaning. Maybe something for a future G1 cleanup.

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Commit messages:
 - Simplify G1CodeBlobClosure

Changes: https://git.openjdk.java.net/loom/pull/112/files
 Webrev: https://webrevs.openjdk.java.net/?repo=loom&pr=112&range=00
  Stats: 12 lines in 4 files changed: 0 ins; 3 del; 9 mod
  Patch: https://git.openjdk.java.net/loom/pull/112.diff
  Fetch: git fetch https://git.openjdk.java.net/loom pull/112/head:pull/112

PR: https://git.openjdk.java.net/loom/pull/112