tensor: Decode rd of macro-op to designate additional accumulator

This is useful when you want to have the tensor core output to multiple
accumulator registers, e.g. when doing outer product within the RF.

hw/rtl/core/VX_decode.sv

@@ -545,6 +545,12 @@ module VX_decode  #(
             `INST_EXT4: begin
                 ex_type = `EX_TENSOR;
                 op_type = `INST_TENSOR_HMMA;
+                // tensor core macroop is encoded as r-type
+                use_rd = 1;
+                `USED_IREG (rd);
+                `USED_IREG (rs1);
+                `USED_IREG (rs2);
+                `USED_IREG (rs3);
             end
         `endif
             default:;