nir: Add is_null_constant to nir_constant

Indicates that the values contained within are 0s, regardless of
type. Enables some optimizations.

Reviewed-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/23173>

src/compiler/nir/nir_lower_io.c

@@ -2530,6 +2530,11 @@ static void
 write_constant(void *dst, size_t dst_size,
                const nir_constant *c, const struct glsl_type *type)
 {
+   if (c->is_null_constant) {
+      memset(dst, 0, dst_size);
+      return;
+   }
+
    if (glsl_type_is_vector_or_scalar(type)) {
       const unsigned num_components = glsl_get_vector_elements(type);
       const unsigned bit_size = glsl_get_bit_size(type);