Over-eager optimization in Pike 8.1

[Per Hedbor () @ Pike (-) developers forum]

F_LOOP is rather simple.

is type int?
  yes:
    is it 0?
     yes: 
      end loop
     no:
      decrement
      call body
  no:
   call f_loop

Note the absence of "or less than" in the check for 0. I guess there
should be such a check. It's there in the interret_functions.h
version.

I will fix it this evening if nobody get's there first. Around line
2854 in amd64.c, for those interreseted. :)