GH-128375: Better instrument for FOR_ITER

[markshannon]

This PR:

• Changes the POP_TOP and the end of a for loop to POP_ITER (it pops the iterator)
• Removes the NOT_TAKEN following FOR_ITER and instruments the FOR_ITER and POP_ITER instead
• Fixes handling of EXTENDED_ARGS in co_branches().

The last one isn't strictly related to the issue, but came up while fixing the main issue.

Changing the instrumentation pattern has two effects:

• For uninstrumented code, there is one less instruction dispatch (the NOT_TAKEN) per iteration, at the cost of one additional dispatch when the loop is exited.
• For instrumented code, once the non-exit branch is DISABLEd, the FOR_ITER can be specialized and jitted.

Performance is neutral on the standard benchmarks for tier1, as expected.

• Issue: Performance of instrumented FOR_ITER is poor as DISABLE doesn't remove the instrumentation. #128375