RFR(XXS) 8224495: Shenandoah: Do not rescan code roots in final mark pause if it is not degenerated GC

[Aleksey Shipilev]

On 5/21/19 6:45 PM, Zhengyu Gu wrote:
> As comments in code stated, it only guards against degenerated GC, that bypasses concurrent cycle.
> So, let's only re-scan code roots if degenerated GC is in progress.
> 
> Bug: https://bugs.openjdk.java.net/browse/JDK-8224495
> Webrev: http://cr.openjdk.java.net/~zgu/JDK-8224495/webrev.00/

Local var "heap" is already available there. Looks okay otherwise.

-Aleksey