GH-123040: Specialize shadowed LOAD_ATTR.

[markshannon]

This PR specializes LOAD_ATTR for shadowed attributes, where the shadowing doesn't change the attribute.

Stats show a 30% reduction in specialization failures for LOAD_ATTR.

Performance might show a slight speedup, but is more likely just noise.

• Issue: Unnecessary specialization failures of LOAD_ATTR and STORE_ATTR when attributes are shadowed by the object's class. #123040

[iritkatriel]

Stats show a 30% reduction in specialization failures for LOAD_ATTR.

Performance might show a slight speedup, but is more likely just noise.

Why is it useful to improve specialisation stats if it's not also improving performance?

[markshannon]

Why is it useful to improve specialisation stats if it's not also improving performance?

It probably does improve performance, and we would expect it to, but it's too small to measure.
That's why we rely on the stats, as they are much less noisy.

We also expect the difference to be magnified on tier 2, once that is more complete.

[iritkatriel]

LGTM, but I think another reviewer might make sense, @brandtbucher or @Fidget-Spinner maybe.

[brandtbucher]

This is some pretty subtle stuff, but it looks correct to me! I wasn't able to make it do anything incorrect with a local build.