nir/range-analysis: Fix incorrect fadd range result for (ne_zero, ne_zero)

Found by inspection.  I tried really, really hard to make a test case
that would trigger this problem, but I was unsuccesful.  It's very hard
to get an instruction to produce a ne_zero result without ne_zero
sources.  The most plausible way is using bcsel.  That proves
problematic because bcsel interprets its sources as integers, so it
cannot currently be used to "clean" values for floating point
instructions.

No shader-db changes on any Intel platform.

Reviewed-by: Caio Marcelo de Oliveira Filho <caio.oliveira@intel.com>
Fixes: 405de7cc ("nir/range-analysis: Rudimentary value range analysis pass")
(cherry picked from commit 0b4782fc)