nir: Introduce a nir_vec_scalars() helper using nir_ssa_scalar.

Many users of nir_vec() do so by nir_channel()-ing a new ssa defs as movs
from other vectors to put the new vector together, which then just have to
get copy-propagated into the ALU srcs and DCEed away the temporary movs.
If they instead take nir_ssa_scalar, we can avoid that extra work.

Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Daniel Schürmann <daniel@schuermann.dev>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/14865>

src/compiler/nir/nir_builder.c

@@ -215,6 +215,32 @@ nir_build_alu_src_arr(nir_builder *build, nir_op op, nir_ssa_def **srcs)
    return nir_builder_alu_instr_finish_and_insert(build, instr);
 }
 
+nir_ssa_def *
+nir_vec_scalars(nir_builder *build, nir_ssa_scalar *comp, unsigned num_components)
+{
+   nir_op op = nir_op_vec(num_components);
+   nir_alu_instr *instr = nir_alu_instr_create(build->shader, op);
+   if (!instr)
+      return NULL;
+
+   for (unsigned i = 0; i < num_components; i++) {
+      instr->src[i].src = nir_src_for_ssa(comp[i].def);
+      instr->src[i].swizzle[0] = comp[i].comp;
+   }
+   instr->exact = build->exact;
+
+   /* Note: not reusing nir_builder_alu_instr_finish_and_insert() because it
+    * can't re-guess the num_components when num_components == 1 (nir_op_mov).
+    */
+   nir_ssa_dest_init(&instr->instr, &instr->dest.dest, num_components,
+                     comp[0].def->bit_size, NULL);
+   instr->dest.write_mask = (1 << num_components) - 1;
+
+   nir_builder_instr_insert(build, &instr->instr);
+
+   return &instr->dest.dest.ssa;
+}
+
 /**
  * Turns a nir_src into a nir_ssa_def * so it can be passed to
  * nir_build_alu()-based builder calls.

src/compiler/nir/nir_builder.h

@@ -371,6 +371,9 @@ nir_vec(nir_builder *build, nir_ssa_def **comp, unsigned num_components)
    return nir_build_alu_src_arr(build, nir_op_vec(num_components), comp);
 }
 
+nir_ssa_def *
+nir_vec_scalars(nir_builder *build, nir_ssa_scalar *comp, unsigned num_components);
+
 static inline nir_ssa_def *
 nir_mov_alu(nir_builder *build, nir_alu_src src, unsigned num_components)
 {