stage2: Add `.save_err_return_trace_index` AIR op

This is encoded as a primitive AIR instruction to resolve one corner
case: A function may include a `catch { ... }` or `else |err| { ... }`
block but not call any errorable fn. In that case, there is no error
return trace to save the index of and codegen needs to avoid
interacting with the non-existing error trace.

By using a primitive AIR op, we can depend on Liveness to mark this
unused in this corner case.

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