nir/glsl_to_nir: use _mesa_fls() to compute num_textures

nir/glsl_to_nir: use _mesa_fls() to compute num_textures

Replace the current loop by a direct call to _mesa_fls() function.

It also fixes an implicit bug in the current code where num_textures
seems to be one value less than it should be when sh->Program->SamplersUsed > 0.

For instance, num_textures is 0 instead of 1 when
sh->Program->SamplersUsed is 1.

Signed-off-by: Juan A. Suarez Romero <jasuarez@igalia.com>
Reviewed-by: Matt Turner <mattst88@gmail.com>