gh-158934: determine if instance or class attribute specialisation is done based off the instance slot value

[johng]

This ensures that we correct specialise class attribute loads even if a slot is defined but not set.

Workload	A/A noise	-O3: main → branch	PGO+LTO: main → branch
queue.Queue put + get	+0.4%	1134 → 1057 ns, −6.7%	983 → 910 ns, −7.4%
Queue producer → consumer thread	−1.3%	1268 → 1192 ns, −6.0%	1142 → 1081 ns, −5.4%
with BoundedSemaphore	−0.8%	852 → 807 ns, −5.4%	726 → 695 ns, −4.3%
threading.Event set + clear	+0.2%	732 → 699 ns, −4.5%	629 → 609 ns, −3.1%
json.dumps tiny status dict	+0.1%	1895 → 1866 ns, −1.5%	1591 → 1537 ns, −3.3%
json.dumps 10-field record	−0.4%	+0.8% (noise)	3592 → 3512 ns, −2.2%
json.dumps 50-record page	−1.5%	−0.9%	−1.0%
HTTP GET + response + json.loads	−1.1%	+0.6%	−1.9%
Future + Queue (mixed site)	−0.2%	−0.5%	+1.3%

• Issue: Specialisation fail LOAD_ATTR_INSTANCE_VALUE specialised when no instance variable #158934