gh-153486: Factor non-negative compact int subscript guards

[KRRT7]

Factor a dedicated non-negative compact-int guard into the specialized subscript/store-subscript fast paths for list, tuple, and str.

This makes the specialization contract explicit and removes redundant checks from the specialized bodies. Negative indices still fall back to the generic path.

Issue: gh-153486

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[KRRT7]

I ran a fresh set of perf checks for this on a Linux VM, rebuilding both main and this branch from scratch and benchmarking pinned to a single CPU (taskset -c 1 + chrt -f 99; pyperf system show reported turbo already disabled).

I used a medium pyperformance subset. The overall picture looks mixed rather than clearly better or worse:

• richards: about 1.05x faster, reproduced on rerun
• nbody: about 1.03x faster, reproduced on rerun
• xml_etree_generate: about 1.02x-1.03x slower, reproduced on rerun

Other differences were either not significant or did not reproduce on rerun (json_dumps, json_loads, pickle, pickle_dict, regex_v8, spectral_norm, xml_etree_parse, xml_etree_iterparse, xml_etree_process).

[KRRT7]

I dug into the negative-index case further and found that the important change here is at specialization time, not just in the specialized opcode body.

With this update, list subscripts using exact compact negative ints can specialize to BINARY_OP_SUBSCR_LIST_INT and stay on the specialized path, instead of remaining on generic BINARY_OP. I verified that on a bare-metal VM: on main, a site like f(L, idx) with idx = -1 stays generic, while with this change the same site specializes to BINARY_OP_SUBSCR_LIST_INT.

I then measured two focused microbenchmarks that exercise that case directly:

1. Negative variable index

L = list(range(100))
idx = -1
x = L[idx]

2. Mixed-sign workload

L = list(range(100))
idxs = (42, -1) * 100

def f(L, idxs):
    s = 0
    for idx in idxs:
        s += L[idx]
    return s

Pinned pyperf runs on that setup gave:

• Negative variable index:

  • main: 13.7 ns +- 1.1 ns
  • patched: 6.66 ns +- 0.46 ns
  • 2.05x faster

• Mixed-sign workload:

  • main: 3.39 us +- 0.14 us
  • patched: 2.36 us +- 0.08 us
  • 1.44x faster

So the win here is specifically for list subscript sites that use negative compact int indices and can now remain specialized.