Use E_AXIS_N where possible

Marlin/src/module/planner.cpp

 
 #if ENABLED(DISTINCT_E_FACTORS)
   uint8_t Planner::last_extruder = 0;     // Respond to extruder change
-  #define _EINDEX (E_AXIS + active_extruder)
-#else
-  #define _EINDEX E_AXIS
 #endif
 
 int16_t Planner::flow_percentage[EXTRUDERS] = ARRAY_BY_EXTRUDERS1(100); // Extrusion factor for each extruder
   position[A_AXIS] = LROUND(a * settings.axis_steps_per_mm[A_AXIS]);
   position[B_AXIS] = LROUND(b * settings.axis_steps_per_mm[B_AXIS]);
   position[C_AXIS] = LROUND(c * settings.axis_steps_per_mm[C_AXIS]);
-  position[E_AXIS] = LROUND(e * settings.axis_steps_per_mm[_EINDEX]);
+  position[E_AXIS] = LROUND(e * settings.axis_steps_per_mm[E_AXIS_N(active_extruder)]);
   #if HAS_POSITION_FLOAT
     position_float[A_AXIS] = a;
     position_float[B_AXIS] = b;
  * Setters for planner position (also setting stepper position).
  */
 void Planner::set_e_position_mm(const float &e) {
+  const uint8_t axis_index = E_AXIS_N(active_extruder);
   #if ENABLED(DISTINCT_E_FACTORS)
-    const uint8_t axis_index = E_AXIS + active_extruder;
     last_extruder = active_extruder;
-  #else
-    const uint8_t axis_index = E_AXIS;
   #endif
   #if ENABLED(FWRETRACT)
     float e_new = e - fwretract.current_retract[active_extruder];
 // Recalculate the steps/s^2 acceleration rates, based on the mm/s^2
 void Planner::reset_acceleration_rates() {
   #if ENABLED(DISTINCT_E_FACTORS)
-    #define AXIS_CONDITION (i < E_AXIS || i == E_AXIS + active_extruder)
+    #define AXIS_CONDITION (i < E_AXIS || i == E_AXIS_N(active_extruder))
   #else
     #define AXIS_CONDITION true
   #endif