[11] 2020-05-28: Bulk backports to sh/jdk11

[Aleksey Shipilev]

https://cr.openjdk.java.net/~shade/shenandoah/backports/jdk11-20200602/webrev.01/

This backports a few easy leftovers to sh/jdk11:
 [backport] 8246162: Shenandoah: full GC does not mark code roots when class unloading is off
 [backport] 8245757: Shenandoah: AlwaysPreTouch should not disable heap resizing or uncommits
 [backport] 8245464: Shenandoah: allocate collection set bitmap at lower addresses
 [backport] 8245773: Shenandoah: Windows assertion failure after JDK-8245464
 [backport] 8245465: Shenandoah: test_in_cset can use more efficient encoding

Testing: hotspot_gc_shenandoah

-- 
Thanks,
-Aleksey