gh-138250: load fast optimization should fall through to empty blocks

[DinoV]

In code like:

def x(tz_str):
    try:
        start, end = 1, 2
    except ValueError as e:
        raise Exception()

    return f(start, end)

The compiler ends up producing an empty basic block. When the load fast optimization runs it sees that term == NULL and doesn't fall through to the following block. The end result is that the compiler doesn't properly produce a LOAD_FAST_BORROWED_LOAD_FAST_BORROWED opcode.

The analysis should just use the BB_HAS_FALLTHROUGH macro instead of duplicating the logic.

• Issue: LOAD_FAST_BORROWED optimization can be missed after an empty block #138250

[mpage]

LGTM - fixes a bug and is much cleaner!