Micro-optimize the LOAD_FAST opcode

[sweeneyde]

The most common opcode before:

TARGET_LOAD_FAST:
    frame->prev_instr = next_instr++;
    PyObject *value = frame->localsplus[oparg]
    if (value == NULL) { goto unbound_local_error; }
    value->ob_refcnt++;
    *stack_pointer++ = value;
    _Py_CODEUNIT word = *next_instr;
    opcode = word & 255;
    oparg = word >> 8;
    opcode |= cframe.use_tracing;
    goto *opcode_targets[opcode];

The most common opcode after:

TARGET_LOAD_FAST_KNOWN_QUICK:
    next_instr++;
    PyObject *value = frame->localsplus[oparg]
    value->ob_refcnt++;
    *stack_pointer++ = value;
    _Py_CODEUNIT word = *next_instr;
    opcode = word & 255;
    oparg = word >> 8;
    goto *opcode_targets[opcode];

In particular:

• The write to frame->prev_instr is removed.
• The NULL-check and branch are removed.
• The memory read and |= are removed.

None of these were particularly significant in isolation, but together, they accounted for an approximately 1% speedup in pyperformance.