dis.py hard-codes that ENTER_EXECUTOR is a backwards jump (it isn't always)

[gvanrossum]

Since we introduced side exits, ENTER_EXECUTOR doesn't always overwrite a JUMP_BACKWARD opcode -- it may overwrite any other opcode too. This requires changes to the logic of ENTER_EXECUTOR.

CC: @iritkatriel

[gvanrossum]

Also, the oparg for ENTER_EXECUTOR is printed as the original oparg, whereas it is typically a different value (the index in the code object's array of executors). E.g. in a function with just one executor I'd expect to see

    ENTER_EXECUTOR    0

but actually I see e.g.

    ENTER_EXECUTOR    13

where 13 is the oparg of the JUMP_BACKWARD it has overwritten.

[iritkatriel]

Duplicate of #112383.