Don't generate guarded devirtualization for abstract classes

[MichalStrehovsky]

Stale PGO data might have information in it suggesting a class that is abstract is a likely class at some virtual callsite (because the class wasn't abstract when the PGO data was collected). Such information is obviously out of date and can be ignored because the devirtualization opportunity will never happen - abstract classes cannot be allocated.

Cc @AndyAyersMS this is tracking what we discussed over email.

Tagging subscribers to this area: @JulieLeeMSFT
See info in area-owners.md if you want to be subscribed.

Issue Details

---

Stale PGO data might have information in it suggesting a class that is abstract is a likely class at some virtual callsite (because the class wasn't abstract when the PGO data was collected). Such information is obviously out of date and can be ignored because the devirtualization opportunity will never happen - abstract classes cannot be allocated.

Cc @AndyAyersMS this is tracking what we discussed over email.

Author:	MichalStrehovsky
Assignees:	-
Labels:	area-CodeGen-coreclr
Milestone:	-

[AndyAyersMS]

Thanks Michal. I have a fix I'll PR shortly.