nir: fix typo in lower_double options handling
Seems the intention was to check that both flags were not enabled instead we were checking that the floor flag was both set and not set so the result would always be false. Fixes: 3749a6ec ("nir: honor lower_double options for ffloor and ffract") Reviewed-by:Gert Wollny <gert.wollny@collabora.com> Part-of: <mesa/mesa!19642>