RFR (S) 8222186: Shenandoah should not uncommit below minimum heap size

[Roman Kennke]

Looks good to me, thanks!

Roman

> RFE:
>   https://bugs.openjdk.java.net/browse/JDK-8222186
> 
> Fix:
>   http://cr.openjdk.java.net/~shade/8222186/webrev.01/
> 
> There is some confusion what -Xms really means. Current code assumes
> it only means "initial" heap
> size, which allows it to uncommit the heap below -Xms. However, users
> expect -Xms to mean "minimum
> heap size" to set the low watermark for heap shrinkage. We need to
> amend Shenandoah to avoid
> uncommitting below that watermark.
> 
> Testing: hotspot_gc_shenandoah (includes new test cases), eyeballing
> gc logs
> 
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