`LOAD_FAST` is not always converted to `LOAD_FAST_BORROW` in a `basicblock`

Bug report

Bug description:

Originally addressed in Issue #144388, the bytecode compiler does not always convert a LOAD_FAST instruction into a LOAD_FAST_BORROW. For example, the following python code, which gets compiled into only one basicblock, has a LOAD_FAST when it could have a LOAD_FAST_BORROW:

from dis import dis

def f(a,b):
    return a if a < b else b

dis(f)

  3           RESUME                   0

  4           LOAD_FAST_BORROW_LOAD_FAST_BORROW 1 (a, b)
              COMPARE_OP              18 (bool(<))
              POP_JUMP_IF_FALSE        3 (to L1)
              NOT_TAKEN
              LOAD_FAST_BORROW         0 (a)
              RETURN_VALUE
      L1:     LOAD_FAST                1 (b)
              RETURN_VALUE

In contrast, the following expanded version of f does convert all LOAD_FAST instructions into LOAD_FAST_BORROW and has three different basic blocks:

from dis import dis

def g(a,b):
    if a < b:
        return a
    else:
        return b

dis(g)

   3           RESUME                   0

  4           LOAD_FAST_BORROW_LOAD_FAST_BORROW 1 (a, b)
              COMPARE_OP              18 (bool(<))
              POP_JUMP_IF_FALSE        3 (to L1)
              NOT_TAKEN

  5           LOAD_FAST_BORROW         0 (a)
              RETURN_VALUE

  7   L1:     LOAD_FAST_BORROW         1 (b)
              RETURN_VALUE

The function of interest is optimize_load_fast in Python/flowgraph.c. I have tried to fix this bug but have not been able to yet. All I have found so far is that the LOAD_FAST instruction is misclassified as REF_UNCONSUMED here for the case of function f:
https://github.com/python/cpython/blob/149c4657507d17f78dd0938419a5a24ed71dc07e/Python/flowgraph.c#L3006-L3011

CPython versions tested on:

CPython main branch

Operating systems tested on:

Linux

Linked PRs

• gh-146503

Start in Python/flowgraph.c at optimize_load_fast, especially the classification around lines 3006–3011. Compare the bytecode produced for the one-basicblock function f and the expanded function g, then verify that the affected LOAD_FAST instruction is converted to LOAD_FAST_BORROW without changing the generated control flow.