RFR: 8221732: Introduce CollectedHeap::hash_oop()

[Kim Barrett]

> On Apr 1, 2019, at 3:39 AM, Per Liden <per.liden at oracle.com> wrote:
> 
> The JVMTITagMap hashes oops by casts them to 32-bit unsigned integers. For ZGC, which has colored oop, this happens to work because all metadata bits are above 32-bit. In theory, ZGC could store its metadata bits in the lower 32-bit part of the address, in which case this hash scheme would break. Today we have an ugly assert in JvmtiTagHashmap::hash() to make sure this doesn't happen. However, the JVMITagMap should not make assumptions on what an oop looks like, and should instead delegate this hashing to CollectedHeap.
> 
> Bug: https://bugs.openjdk.java.net/browse/JDK-8221732
> Webrev: http://cr.openjdk.java.net/~pliden/8221732/webrev.0
> 
> /Per

Looks good.