qiskit: Remove save_state instructions when transpiling to backend that does not support them

What is the expected enhancement?

Summary: When transpiling a circuit with save_state instructions (such as those inserted into a circuit with the `save_statevector() function) to a backend that does not support it, remove/ignore them.

Users can save the statevector at the midpoint of a circuit by calling the save_statevector() function, which is useful for probing the circuit during simulations. However, then transpiling the same circuit to run on a backend that does not support the save_state instruction, such as the real quantum computer backends, the transpiler will fail with the message like this:

Unable to map source basis {('measure', 1), ('save_state', 1), ('h', 1)} to target basis {'reset', 'rz', 'measure', 'barrier', 'delay', 'x', 'snapshot', 'id', 'sx', 'cx'}

This is may be confusing the the user. Assuming that they interpret this error message correctly, their recourse is to regenerate the source circuit without the save_state instructions (e.g. by removing calls to save_statevector()) and transpiling again. This is cumbersome and perhaps error prone.

An improved approach would be for the transpiler to simply remove/skip/ignore save_state instructions when the target backend does not support them. The benefit of this is that it would allow the user to switch easily between statevector simulators and real quantum computer backends.