Combine SAVE_IP and SAVE_CURRENT_IP into a new uop

[gvanrossum]

See #621 (comment):

Now, there's an explicit SAVE_CURRENT_IP in some macros (before every _POP_FRAME and _PUSH_FRAME). That's because it also acts as a flag for the code generator, which, combined with the preceding SAVE_IP, makes it effectively save a pointer to the next instruction (which is what's needed by the frame push/pop uops). But there's so much special-casing here that we might as well either introduce a new uop for the combined special effects or special-case _POP_FRAME and _PUSH_FRAME.

Also rename SAVE_IP to SET_IP (from the same issue).

[gvanrossum]

Sorry, meant to create this in cpython. See python/cpython#109214.