Why use a Mux to reset a register, instead of creating a CompRegOp with the reset signal and the reset value?

[XunChangqing]

This results in Arcilator being unable to perform effective reset grouping in the GroupResetAndEnable pass.

      // Canonicalize registers.
      if (auto reg = dyn_cast<seq::FirRegOp>(&op)) {
        OpBuilder builder(reg);
        Value next;
        // Note: this register will have an sync reset regardless.
        if (reg.hasReset())
          next = comb::MuxOp::create(builder, reg.getLoc(), reg.getReset(),
                                     reg.getResetValue(), reg.getNext(), false);
        else
          next = reg.getNext();

        Value presetValue;
        // Materialize initial value, assume zero initialization as default.
        if (reg.getPreset() && !reg.getPreset()->isZero()) {
          assert(hw::type_isa<IntegerType>(reg.getType()) &&
                 "cannot lower non integer preset");
          presetValue = circt::seq::createConstantInitialValue(
              builder, reg.getLoc(),
              IntegerAttr::get(reg.getType(), *reg.getPreset()));
        }

        Value compReg = seq::CompRegOp::create(
            builder, reg.getLoc(), next.getType(), next, reg.getClk(),
            reg.getNameAttr(), Value{}, Value{}, /*initialValue*/ presetValue,
            reg.getInnerSymAttr());
        reg.replaceAllUsesWith(compReg);
        opsToDelete.push_back(reg);
        continue;
      }