Documenation in ifcvt.c

[Richard Kenner]

    The patch that I approved avoids sprinkling lots and lots
    of copy_rtx calls all over the place.  I rejected that idea
    because it's very error-prone to miss a place that would 
    need such attention.

Yup.  I'm not in love with my approach either and I was trying to understand
the code better to figure out a more precise patch, which was why I posted
about the documentation.