[8] RFR: Fix slowdebug build after JDK-8230853 backport

[Roman Kennke]

Ok.

Thanks,
Roman

> CI reports slowdebug build failure after recent JDK-8230853 backport.
> 
> It is 8u-specific, because it has the assert that takes in_collection_set(HeapWord*) that is not
> available anymore. The fix is to ditch the assert and get that code in line with what sh/jdk11 and
> jdk/jdk are doing:
> 
> diff -r b49cd4f04b3c src/share/vm/gc_implementation/shenandoah/shenandoahHeap.cpp
> --- a/src/share/vm/gc_implementation/shenandoah/shenandoahHeap.cpp      Mon Apr 06 20:35:43 2020 +0200
> +++ b/src/share/vm/gc_implementation/shenandoah/shenandoahHeap.cpp      Wed Apr 22 11:48:28 2020 +0200
> @@ -824,13 +824,7 @@
>  HeapWord*  ShenandoahHeap::mem_allocate(size_t size,
>                                          bool*  gc_overhead_limit_was_exceeded) {
>    ShenandoahAllocRequest req = ShenandoahAllocRequest::for_shared(size);
> -  HeapWord* result = allocate_memory(req);
> -  if (result != NULL) {
> -    assert(! in_collection_set(result), "never allocate in targetted region");
> -    return result;
> -  } else {
> -    return NULL;
> -  }
> +  return allocate_memory(req);
>  }
> 
>  class ShenandoahConcurrentEvacuateRegionObjectClosure : public ObjectClosure {
> 
> 
> Testing: Linux x86_64 {release,fastdebug,slowdebug} builds
>