gh-130279: Fix missing exception handlers for jumps in loops

[LH-and-FPGA]

When a loop body ends with an if whose false branch jumps back to the loop head, the backward jump ended up outside the exception table ranges of the enclosing try/with. An exception raised when that JUMP_BACKWARD checks the eval breaker (e.g. KeyboardInterrupt, or an async exception set via PyThreadState_SetAsyncExc) then skipped the except/finally blocks, and __exit__ was not called for with blocks.

def scan(items):
    try:
        for x in items:
            if x < 0:
                raise ValueError(x)
    except KeyboardInterrupt:
        return "handled"

Two places in Python/flowgraph.c created jump instructions without an exception handler after label_exception_targets() had already run:

• jump_thread() replaced the original jump with a new one, dropping its i_except.
• normalize_jumps_in_block() added NOT_TAKEN + JUMP for backward conditional jumps without copying the handler.

Both now copy the handler of the original jump.

• Issue: KeyboardInterrupt not handled while evaluating a tight while loop condition on Python 3.13 #130279

[python-cla-bot]

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