RFR (S): [9+8u] Min region size should be 256K to allow small heaps

[Aleksey Shipilev]

Hi,

Turns out, some JDK tool launchers set -Xms8M. Shenandoah fails cryptically then:

Error occurred during initialization of VM
Invalid -XX:ShenandoahMinRegionSize option

While those tools configs are the actual culprit, we might want to bite the
bullet and afford those smaller heaps. And turns out, we supposed to! Because
the failure is due to requesting 10 regions, with SHR::MIN_REGION_SIZE being
256K. But that ShenandoahMinRegionSize default is 1M, oops. I suggest we change
ShenandoahMinRegionSize to match MIN_REGION_SIZE:
  http://cr.openjdk.java.net/~shade/shenandoah/regionsizes-match/webrev.01/

I don't want to depend on ShenandoahHeapRegion in *globals.hpp. Tests should
cover future discrepancies.

The patch applies to 8u with some fuzz, and I will push it there simultaneously
with 9.

Testing: hotspot_gc_shenandoah

Thanks,
-Aleksey