RFR (XS) 8245757: Shenandoah: AlwaysPreTouch should not disable heap resizing or uncommits

[Aleksey Shipilev]

RFE:
  https://bugs.openjdk.java.net/browse/JDK-8245757

At present, enabling -XX:+AlwaysPreTouch implicitly disables uncommit and sets the heap size to Xmx.
This is not coherent with the rest of OpenJDK garbage collectors that only pretouch when the actual
commit happens. Shenandoah should do the same. It would still be recommended to set -Xmx == -Xms for
AlwaysPreTouch to work without excess latencies.

Webrev:
  https://cr.openjdk.java.net/~shade/8245757/webrev.01/

Testing: hotspot_gc_shenandoah; ad-hoc perf runs

-- 
Thanks,
-Aleksey