RFR: Use single-condition instance type checks

[Thomas Schatzl]

On Wed, 23 Mar 2022 10:41:41 GMT, Stefan Karlsson <stefank at openjdk.org> wrote:

> Loom requires specialized code for stackChunks objects when marking/copying objects. This adds extra checks for every visited object. The current implementation checks both the layout helper, then checks the _kind flag. I suggest that we reimplement this as a single check against the Klass::_id field.
> 
> This change would probably be a good cleanup for the mainline.

Marked as reviewed by tschatzl (no project role).

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PR: https://git.openjdk.java.net/loom/pull/118