RFR: 8305062: Refactor CardTable::resize_covered_region [v3]

[Albert Mingkun Yang]

> Simple refactoring to make logic around cardtable cover-region more concrete, since #generations and gen-boundary is fixed for Serial/Parallel.
> 
> Test: tier1-6

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/13206/files
  - new: https://git.openjdk.org/jdk/pull/13206/files/430acad6..c2e05f8e

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=13206&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=13206&range=01-02

  Stats: 3 lines in 1 file changed: 0 ins; 0 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/13206.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/13206/head:pull/13206

PR: https://git.openjdk.org/jdk/pull/13206