nir: Fill in missing conversion opts

I noticed we were missing:

    (('u2f16', ('u2u64', 'a@32')), ('u2f16', a))

This was do to coupling the u2f/i2f opts with i2i/u2u in the same loop
(with different positionals).  The `if B <= S\ncontinue` doesn't apply
to the second part.  So just split these into two loops.

Closes: https://gitlab.freedesktop.org/mesa/mesa/-/issues/14848
Signed-off-by: Rob Clark <rob.clark@oss.qualcomm.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/39899>