Stop inlining code to convert integers to floats.

Stop inlining code to convert integers to floats.

Do the conversion by calling .cif8 or .cuf8 in libem, as it was done
before my commit 1de1e8f.  I used the inline conversion to experiment
with the register allocator, which was too slow until c5bb3be.

Now that libem has the only copy of the code, move some comments and
code changes there.