RFR: 8274988 G1: refine G1SegmentedArrayAllocOptions and G1CardSetAllocOptions

[Hamlin Li]

This is a follow-up of JDK-8273626. (something mentioned in Description was already done in JDK-8273626)
Some other cleanups need to be done:
Some fields of G1SegmentedArrayAllocOptions better be reloated to G1CardSetAllocOptions, and some fields are const, and also adds some assert.

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Commit messages:
 - Initial commit

Changes: https://git.openjdk.java.net/jdk/pull/6032/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=6032&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8274988
  Stats: 16 lines in 2 files changed: 8 ins; 4 del; 4 mod
  Patch: https://git.openjdk.java.net/jdk/pull/6032.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/6032/head:pull/6032

PR: https://git.openjdk.java.net/jdk/pull/6032