Experiment with putting most of potential_passthrough_impl in an cold function

[hsivonen]

potential_passthrough_impl starts with the remark

// This methods looks badly branchy, but most characters
// take the first return.

Experiment with delegating the rest of the function after the first check to a second function marked cold.

[sffc]

@hsivonen This looks like internal cleanup, correct? If it impacts data struct stability, we could do it sooner.

[hsivonen]

This is an internal change, yes.