GH-145667: Merge GET_ITER and GET_YIELD_FROM_ITER

[markshannon]

Also modifies SEND to make it a bit more like FOR_ITER as both are now fed by GET_ITER.

Apart from saving an opcode, this will enable broadly similar optimizations to be applied to SEND as are already applied to FOR_ITER. Which should:

• significantly improve the performance of yield from seq where seq is a common iterable, like list.
• allow the JIT to trace through SEND as it will be mostly specialized.

For now, this is broadly performance neutral, between +0% and -1%, mostly about 0%.

• Issue: Merge the GET_ITER and GET_YIELD_FROM_ITER instructions #145667