rpeep() assert oldoldop -> oldop -> o form a chain

rpeep() assert oldoldop -> oldop -> o form a chain

In rpeep(), in a loop, the var o becomes each op in the op_next chain in
turn. At the same time, oldop is set to the previous value of o, and
oldoldop the previous but one.

Some places that modify the op_next chain weren't correctly upodating
oldop and oldoldop at the same time. Last few commits have fixed those
places.

This commit adds an assert at the top of loop to check that oldoldop and
oldop are in fact consistent.

(This assert was used to find the faults fixed in the previous couple of
commits).