[8] RFR: 2020-05-19, Bulk backports to sh/jdk8

[Aleksey Shipilev]

https://cr.openjdk.java.net/~shade/shenandoah/backports/jdk8u-20200519/webrev.01/

This backports a few easy leftovers, mostly file moves to sh/jdk8:
 [backport] 8241062: Shenandoah: rich asserts trigger "empty statement" inspection
 [backport] 8244730: Shenandoah: gc/shenandoah/options/TestHeuristicsUnlock.java should only verify
 [backport] 8244732: Shenandoah: move heuristics code to gc/shenandoah/heuristics
 [backport] 8244737: Shenandoah: move mode code to gc/shenandoah/mode
 [backport] 8244739: Shenandoah: break superclass dependency on ShenandoahNormalMode
 [backport] 8244740: Shenandoah: rename ShenandoahNormalMode to ShenandoahSATBMode

Testing: hotspot_gc_shenandoah {fastdebug,release}

-- 
Thanks,
-Aleksey