loop: remove code size-estimation.

Loop has a facility to determine whether it's ok to duplicate variable
initialization and stepping code when the variable preceding it has
different initialization and stepping forms. The code which determines
code size is quite strange and it may have been relevant 20 years ago
on primitive implementations, but not anymore, and people who really
care about code size would use functions, which will also improve code
readability.

As a side effect, it fixes a bug which was present in the
estimate-code-size function.

Fixes lp#1178989.