Refactor `RETURN_VALUE` and `RETURN_CONST`

[markshannon]

The RETURN_VALUE and RETURN_CONST instructions return the value on top of the stack and a constant, respectively.
RETURN_CONST is effectively a superinstruction: LOAD_CONST + RETURN_VALUE.

This worked well up to 3.13, but is likely to be an impediment to further optimizations:

• The JIT already breaks RETURN_CONST into LOAD_CONST and RETURN_VALUE so the superinstruction has no value.
• RETURN_VALUE has two different paths internally, one for normal functions and the other for generators. We know statically which sort of function we are in, so we should emit different bytecodes for the different behavior
• With the proposed changes to reference counting, it will help in the interpreter (we already do this in the JIT) to have a different instruction for immortal constants. (This might only be temporarily, as we expect to be able to defer reference counting on all constants)

Changes to the instruction set:

• RETURN_CONST and INSTRUMENTED_RETURN_CONST will be removed.
• RETURN_VALUE_GEN, INSTRUMENTED_RETURN_GEN and LOAD_CONST_IMMORTAL will be added.

We expect that LOAD_CONST_IMMORTAL will be removed again before 3.14, but that is not a requirement.

Linked PRs

• GH-125837: Rework the instructions for loading constants and returning values. #125878
• GH-125837: Split LOAD_CONST into three. #125972