8265239: Shenandoah: Shenandoah heap region count could be off by 1

Backport-of: ff5bb8cf693c58a14063a351f535e3a55e51e8db

src/hotspot/share/gc/shenandoah/shenandoahHeap.cpp

@@ -151,6 +151,7 @@ jint ShenandoahHeap::initialize() {
   Universe::check_alignment(init_byte_size, reg_size_bytes, "Shenandoah heap");
 
   _num_regions = ShenandoahHeapRegion::region_count();
+  assert(_num_regions == (max_byte_size / reg_size_bytes), "Must match");
 
   // Now we know the number of regions, initialize the heuristics.
   initialize_heuristics();

src/hotspot/share/gc/shenandoah/shenandoahHeapRegion.cpp

@@ -573,7 +573,7 @@ void ShenandoahHeapRegion::setup_sizes(size_t max_heap_size) {
   RegionSizeBytesMask = RegionSizeBytes - 1;
 
   guarantee(RegionCount == 0, "we should only set it once");
-  RegionCount = max_heap_size / RegionSizeBytes;
+  RegionCount = align_up(max_heap_size, RegionSizeBytes) / RegionSizeBytes;
   guarantee(RegionCount >= MIN_NUM_REGIONS, "Should have at least minimum regions");
 
   guarantee(HumongousThresholdWords == 0, "we should only set it once");