nir: Return NULL, not false, from functions returning pointers

Reported by clang's `-Wbool-conversion`.

Reviewed-by: Caio Oliveira <caio.oliveira@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/34014>

src/compiler/nir/nir_lower_double_ops.c

@@ -545,14 +545,14 @@ lower_doubles_instr_to_soft(nir_builder *b, nir_alu_instr *instr,
    switch (instr->op) {
    case nir_op_f2i64:
       if (instr->src[0].src.ssa->bit_size != 64)
-         return false;
+         return NULL;
       name = "__fp64_to_int64";
       mangled_name = "__fp64_to_int64(u641;";
       return_type = glsl_int64_t_type();
       break;
    case nir_op_f2u64:
       if (instr->src[0].src.ssa->bit_size != 64)
-         return false;
+         return NULL;
       name = "__fp64_to_uint64";
       mangled_name = "__fp64_to_uint64(u641;";
       break;
@@ -675,7 +675,7 @@ lower_doubles_instr_to_soft(nir_builder *b, nir_alu_instr *instr,
       return_type = glsl_bool_type();
       break;
    default:
-      return false;
+      return NULL;
    }
 
    assert(softfp64 != NULL);

src/compiler/nir/nir_opt_intrinsics.c

@@ -64,7 +64,7 @@ try_opt_bcsel_of_shuffle(nir_builder *b, nir_alu_instr *alu,
     * now and subgroup ops in the presence of discard aren't common.
     */
    if (block_has_discard)
-      return false;
+      return NULL;
 
    if (!nir_alu_src_is_trivial_ssa(alu, 0))
       return NULL;

src/compiler/nir/nir_opt_varyings.c

@@ -2814,12 +2814,12 @@ check_tes_input_load_get_single_use_alu(nir_intrinsic_instr *load,
 
    /* Check the vertex index. Each vertex can be loaded only once. */
    if (!nir_src_is_const(load->src[0]))
-      return false;
+      return NULL;
 
    *vertex_index = nir_src_as_uint(load->src[0]);
    if (*vertex_index >= max_vertices ||
        *vertices_used & BITFIELD_BIT(*vertex_index))
-      return false;
+      return NULL;
 
    *vertices_used |= BITFIELD_BIT(*vertex_index);