RFR (XS): Heap alignment should agree with ShenandoahMaxRegionSize

[Aleksey Shipilev]

Hi,

Tried to use 1G regions with 4T heap, and fastdebug failed with

#  assert((((size_t) _first_region_bottom) &
(ShenandoahHeapRegion::RegionSizeBytes - 1)) == 0) failed: misaligned heap:
0x00007b05f4000000

This is because CollectedHeap::conservative_heap_alignment() returns 32M, not
the value overridden from the command line! And, we poll the value from G1, not
from our own ShenandoahHeapRegion! Remarkably, in 8u, we return
ShenandoahMaxRegionSize.

This is the fix for 9:
  http://cr.openjdk.java.net/~shade/shenandoah/regionsizes-heap-align/webrev.01/

Moved both constants close to verification code that their only use. Will commit
a part of this patch to 8u that does the same.

Testing: hotspot_gc_shenandoah (9/8u), 1G regions with 4T heap startup :)

Thanks,
-Aleksey