effects: pack bits better (#56737)
There is no reason to preserve duplicates of the bits for the value
before and after inference, and many of the numbers in the comments had
gotten incorrect. Now we are able to pack all 16 bits of currently
defined bitflags into 20 bits, instead of 25 bits (although either case
still rounds up to 32).
There was also no reason for InferenceState to be mutating of CodeInfo
during execution, so remove that mutation.
