RFR: 8290376: G1: Refactor G1MMUTracker::when_sec [v2]

[Albert Mingkun Yang]

> Introducing a more intuitive implementation.
> 
> This PR consists of two commits: the first adds the new algorithm and verifies the result is same as before. The second commit removes the old implementation.
> 
> Test: tier1-6 for the first commit.

Albert Mingkun Yang has updated the pull request incrementally with one additional commit since the last revision:

  review

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Changes:
  - all: https://git.openjdk.org/jdk/pull/9517/files
  - new: https://git.openjdk.org/jdk/pull/9517/files/78a64165..a52a6f54

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=9517&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=9517&range=00-01

  Stats: 29 lines in 1 file changed: 6 ins; 0 del; 23 mod
  Patch: https://git.openjdk.org/jdk/pull/9517.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/9517/head:pull/9517

PR: https://git.openjdk.org/jdk/pull/9517