RFR: 8265239: Shenandoah: Shenandoah heap region count could be 1 short

[Zhengyu Gu]

henandoah uses MaxHeapSize to calculate region size, then aligns up heap size to region size alignment, but when it calculates region count, it does not take alignment into account, that may result 1 region short than actual count.

MaxHeapSize eventually aligns to HeapAlignment (which is region size in Shenandoah case) in GCArguments::initialize_heap_flags_and_sizes(), but it is too late.

Test:
- [x] hotspot_gc_shenandoah

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Commit messages:
 - JDK-8265239

Changes: https://git.openjdk.java.net/jdk/pull/3504/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=3504&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8265239
  Stats: 2 lines in 2 files changed: 1 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/3504.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/3504/head:pull/3504

PR: https://git.openjdk.java.net/jdk/pull/3504