nir: Add a nir_src_is_undef() helper, like nir_src_is_const().

Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/9345>

src/compiler/nir/nir.h

@@ -970,6 +970,13 @@ nir_src_is_const(nir_src src)
           src.ssa->parent_instr->type == nir_instr_type_load_const;
 }
 
+static inline bool
+nir_src_is_undef(nir_src src)
+{
+   return src.is_ssa &&
+          src.ssa->parent_instr->type == nir_instr_type_ssa_undef;
+}
+
 static inline bool
 nir_src_is_divergent(nir_src src)
 {

src/compiler/nir/nir_divergence_analysis.c

@@ -746,7 +746,7 @@ visit_loop_header_phi(nir_phi_instr *phi, nir_block *preheader, bool divergent_c
       if (src->pred == preheader)
          continue;
       /* skip undef values */
-      if (src->src.ssa->parent_instr->type == nir_instr_type_ssa_undef)
+      if (nir_src_is_undef(src->src))
          continue;
 
       /* check if all loop-carried values are from the same ssa-def */

src/compiler/nir/nir_gather_ssa_types.c

@@ -72,8 +72,7 @@ static void
 copy_types(nir_src src, nir_dest *dest, BITSET_WORD *float_types,
            BITSET_WORD *int_types, bool *progress)
 {
-   bool src_is_sink = nir_src_is_const(src) ||
-                      src.ssa->parent_instr->type == nir_instr_type_ssa_undef;
+   bool src_is_sink = nir_src_is_const(src) || nir_src_is_undef(src);
    copy_type(src.ssa->index, dest->ssa.index, src_is_sink, float_types, progress);
    copy_type(src.ssa->index, dest->ssa.index, src_is_sink, int_types, progress);
 }

src/compiler/nir/nir_liveness.c

@@ -77,7 +77,7 @@ set_src_live(nir_src *src, void *void_live)
    if (!src->is_ssa)
       return true;
 
-   if (src->ssa->parent_instr->type == nir_instr_type_ssa_undef)
+   if (nir_src_is_undef(*src))
       return true;   /* undefined variables are never live */
 
    BITSET_SET(live, src->ssa->index);

src/compiler/nir/nir_opt_remove_phis.c

@@ -98,7 +98,7 @@ remove_phis_block(nir_block *block, nir_builder *b)
          if (def == NULL) {
             def  = src->src.ssa;
             mov = get_parent_mov(def);
-         } else if (src->src.ssa->parent_instr->type == nir_instr_type_ssa_undef &&
+         } else if (nir_src_is_undef(src->src) &&
                     nir_block_dominates(def->parent_instr->block, src->pred)) {
             /* Ignore this undef source. */
          } else {

src/intel/compiler/brw_fs_nir.cpp

@@ -1918,7 +1918,7 @@ fs_visitor::get_nir_src(const nir_src &src)
 {
    fs_reg reg;
    if (src.is_ssa) {
-      if (src.ssa->parent_instr->type == nir_instr_type_ssa_undef) {
+      if (nir_src_is_undef(src)) {
          const brw_reg_type reg_type =
             brw_reg_type_from_bit_size(src.ssa->bit_size, BRW_REGISTER_TYPE_D);
          reg = bld.vgrf(reg_type, src.ssa->num_components);