RFR (XS) [9+8u] Explicit ShenandoahHeapRegionSize should be checked against min/max

[Aleksey Shipilev]

The flip side of heap alignment depending on ShenandoahMaxRegionSize is that we
cannot allow explicit ShenandoahHeapRegionSize to cross it. It is never checked
if explicit.

Fix:
  http://cr.openjdk.java.net/~shade/shenandoah/regionsizes-forceHRS/webrev.01/

Testing: hotspot_gc_shenandoah (9/8u)

Thanks,
-Aleksey