add S_process_optree() function to op.c

add S_process_optree() function to op.c

Extract into a new static function, S_process_optree(), some of the common
code that calls rpeep(), finalize_optree() etc in the various
newSUB()-style functions.

There should be no functional changes, except that for formats, pad_tidy()
is now called *after* optimisation. This matches what already happens for
normals subs, and can possibly be regarded as a bug fix - although I can't
think of anything it actually fixes.

There are probably more things that could be consolidated into this
function, but because the details vary amongst the various call sites,
I've left them alone for now.