r600/sfn: use three channels only for unary trans opts if possible

Signed-off-by: Gert Wollny <gert.wollny@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/20141>

src/gallium/drivers/r600/sfn/sfn_instr_alu.cpp

@@ -2819,12 +2819,10 @@ emit_alu_trans_op1_cayman(const nir_alu_instr& alu, EAluOp opcode, Shader& shade
 
    auto pin = pin_for_components(alu);
 
-   unsigned ncomp = 4; //nir_dest_num_components(alu.dest.dest) == 4 ? 4 : 3;
-
-   /* todo: Actually we need only three channels, but then we have
-    * to make sure that we don't hava w dest */
-   for (unsigned j = 0; j < ncomp; ++j) {
+   for (unsigned j = 0; j < nir_dest_num_components(alu.dest.dest); ++j) {
       if (alu.dest.write_mask & (1 << j)) {
+         unsigned ncomp =  j == 3 ? 4 : 3;
+
          AluInstr::SrcValues srcs(ncomp);
          PRegister dest = value_factory.dest(alu.dest.dest, j, pin, (1 << ncomp) - 1);