nir: replace nir_opt_remove_phis_block with a single source version

This is what callers actually want, and it simplifies nir_opt_remove_phis
because we can assume dominance meta data is valid.

Reviewed-by: Daniel Schürmann <daniel@schuermann.dev>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/31031>

src/amd/compiler/aco_instruction_selection_setup.cpp

@@ -88,7 +88,7 @@ sanitize_if(nir_function_impl* impl, nir_if* nif)
     * or dead control-flow passes and are perfectly legal.  Run a quick phi
     * removal on the block after the if to clean up any such phis.
     */
-   nir_opt_remove_phis_block(nir_cf_node_as_block(nir_cf_node_next(&nif->cf_node)));
+   nir_remove_single_src_phis_block(nir_cf_node_as_block(nir_cf_node_next(&nif->cf_node)));
 
    /* Finally, move the continue from branch after the if-statement. */
    nir_block* last_continue_from_blk = then_jump ? else_block : then_block;

src/compiler/nir/nir.h

@@ -6817,7 +6817,7 @@ bool nir_opt_reassociate_bfi(nir_shader *shader);
 bool nir_opt_rematerialize_compares(nir_shader *shader);
 
 bool nir_opt_remove_phis(nir_shader *shader);
-bool nir_opt_remove_phis_block(nir_block *block);
+bool nir_remove_single_src_phis_block(nir_block *block);
 
 bool nir_opt_phi_precision(nir_shader *shader);
 

src/compiler/nir/nir_opt_if.c

@@ -766,7 +766,7 @@ opt_if_simplification(nir_builder *b, nir_if *nif)
        */
       nir_block *const next_block =
          nir_cf_node_as_block(nir_cf_node_next(&nif->cf_node));
-      nir_opt_remove_phis_block(next_block);
+      nir_remove_single_src_phis_block(next_block);
    }
 
    rewrite_phi_predecessor_blocks(nif, then_block, else_block, else_block,

src/compiler/nir/nir_opt_loop.c

@@ -162,7 +162,7 @@ opt_loop_terminator(nir_if *nif)
     * or dead control-flow passes and are perfectly legal.  Run a quick phi
     * removal on the block after the if to clean up any such phis.
     */
-   nir_opt_remove_phis_block(nir_cf_node_as_block(nir_cf_node_next(&nif->cf_node)));
+   nir_remove_single_src_phis_block(nir_cf_node_as_block(nir_cf_node_next(&nif->cf_node)));
 
    /* Finally, move the continue from branch after the if-statement. */
    nir_cf_list tmp;
@@ -266,7 +266,7 @@ opt_loop_last_block(nir_block *block, bool is_trivial_continue, bool is_trivial_
          continue;
 
       /* If there are single-source phis after the IF, get rid of them first */
-      nir_opt_remove_phis_block(nir_cf_node_cf_tree_next(prev));
+      nir_remove_single_src_phis_block(nir_cf_node_cf_tree_next(prev));
 
       /* We are about to remove one predecessor. */
       nir_lower_phis_to_regs_block(block->successors[0]);
@@ -405,7 +405,7 @@ opt_loop_peel_initial_break(nir_loop *loop)
     * or dead control-flow passes and are perfectly legal.  Run a quick phi
     * removal on the block after the if to clean up any such phis.
     */
-   nir_opt_remove_phis_block(nir_cf_node_cf_tree_next(if_node));
+   nir_remove_single_src_phis_block(nir_cf_node_cf_tree_next(if_node));
 
    /* We need LCSSA because we are going to wrap the loop into an IF. */
    nir_convert_loop_to_lcssa(loop);

src/compiler/nir/nir_opt_remove_phis.c

@@ -133,10 +133,27 @@ remove_phis_block(nir_block *block, nir_builder *b)
 }
 
 bool
-nir_opt_remove_phis_block(nir_block *block)
+nir_remove_single_src_phis_block(nir_block *block)
 {
-   nir_builder b = nir_builder_create(nir_cf_node_get_function(&block->cf_node));
-   return remove_phis_block(block, &b);
+   assert(block->predecessors->entries <= 1);
+   bool progress = false;
+   nir_foreach_phi_safe(phi, block) {
+      nir_def *def = NULL;
+      nir_foreach_phi_src(src, phi) {
+         def = src->src.ssa;
+         break;
+      }
+
+      if (!def) {
+         nir_builder b = nir_builder_create(nir_cf_node_get_function(&block->cf_node));
+         b.cursor = nir_after_phis(block);
+         def = nir_undef(&b, phi->def.num_components, phi->def.bit_size);
+      }
+
+      nir_def_replace(&phi->def, def);
+      progress = true;
+   }
+   return progress;
 }
 
 static bool