LIN_ADVANCE bug fix and optimization

.) long to int: Extruder stalls at 10kHz / 20kHz step limits with long.
.) Take the delta_adv_steps calculation out of the step_loops loop. Wasted calculation performance if done inside.
.) >> 2 replaced by 3: Is divide by 8. Reason: Timer 0 runs at 16/8=2MHz, Timer 1 at 16/64=0.25MHz. ==> 2/0.25=8.

Marlin/stepper.cpp

   volatile unsigned char Stepper::eISR_Rate = 200; // Keep the ISR at a low rate until needed
 
   #if ENABLED(LIN_ADVANCE)
-    volatile long Stepper::e_steps[E_STEPPERS];
+    volatile int Stepper::e_steps[E_STEPPERS];
     int Stepper::extruder_advance_k = LIN_ADVANCE_K,
         Stepper::final_estep_rate,
         Stepper::current_estep_rate[E_STEPPERS],
 
       #if ENABLED(MIXING_EXTRUDER)
         // Step mixing steppers proportionally
-        bool dir = motor_direction(E_AXIS);
+        const bool dir = motor_direction(E_AXIS);
         MIXING_STEPPERS_LOOP(j) {
           counter_m[j] += current_block->steps[E_AXIS];
           if (counter_m[j] > 0) {
         }
       #endif
 
-      if (current_block->use_advance_lead) {
-        int delta_adv_steps = (((long)extruder_advance_k * current_estep_rate[TOOL_E_INDEX]) >> 9) - current_adv_steps[TOOL_E_INDEX];
-        #if ENABLED(MIXING_EXTRUDER)
-          // Mixing extruders apply advance lead proportionally
-          MIXING_STEPPERS_LOOP(j) {
-            int steps = delta_adv_steps * current_block->step_event_count / current_block->mix_event_count[j];
-            e_steps[j] += steps;
-            current_adv_steps[j] += steps;
-          }
-        #else
-          // For most extruders, advance the single E stepper
-          e_steps[TOOL_E_INDEX] += delta_adv_steps;
-          current_adv_steps[TOOL_E_INDEX] += delta_adv_steps;
-        #endif
-      }
-
     #elif ENABLED(ADVANCE)
 
       // Always count the unified E axis
       #if ENABLED(MIXING_EXTRUDER)
 
         // Step mixing steppers proportionally
-        bool dir = motor_direction(E_AXIS);
+        const bool dir = motor_direction(E_AXIS);
         MIXING_STEPPERS_LOOP(j) {
           counter_m[j] += current_block->steps[E_AXIS];
           if (counter_m[j] > 0) {
     }
   }
 
+  #if ENABLED(LIN_ADVANCE)
+    if (current_block->use_advance_lead) {
+      int delta_adv_steps = (((long)extruder_advance_k * current_estep_rate[TOOL_E_INDEX]) >> 9) - current_adv_steps[TOOL_E_INDEX];
+      current_adv_steps[TOOL_E_INDEX] += delta_adv_steps;
+      #if ENABLED(MIXING_EXTRUDER)
+        // Mixing extruders apply advance lead proportionally
+        MIXING_STEPPERS_LOOP(j)
+          e_steps[j] += delta_adv_steps * current_block->step_event_count / current_block->mix_event_count[j];
+      #else
+        // For most extruders, advance the single E stepper
+        e_steps[TOOL_E_INDEX] += delta_adv_steps;
+      #endif
+   }
+  #endif
+  
   #if ENABLED(ADVANCE) || ENABLED(LIN_ADVANCE)
     // If we have esteps to execute, fire the next advance_isr "now"
     if (e_steps[TOOL_E_INDEX]) OCR0A = TCNT0 + 2;
     #endif // ADVANCE or LIN_ADVANCE
 
     #if ENABLED(ADVANCE) || ENABLED(LIN_ADVANCE)
-      eISR_Rate = (timer >> 2) * step_loops / abs(e_steps[TOOL_E_INDEX]);
+      eISR_Rate = (timer >> 3) * step_loops / abs(e_steps[TOOL_E_INDEX]); //>> 3 is divide by 8. Reason: Timer 0 runs at 16/8=2MHz, Timer 1 at 16/64=0.25MHz. ==> 2/0.25=8.
     #endif
   }
   else if (step_events_completed > (uint32_t)current_block->decelerate_after) {
     #endif // ADVANCE or LIN_ADVANCE
 
     #if ENABLED(ADVANCE) || ENABLED(LIN_ADVANCE)
-      eISR_Rate = (timer >> 2) * step_loops / abs(e_steps[TOOL_E_INDEX]);
+      eISR_Rate = (timer >> 3) * step_loops / abs(e_steps[TOOL_E_INDEX]);
     #endif
   }
   else {
       if (current_block->use_advance_lead)
         current_estep_rate[TOOL_E_INDEX] = final_estep_rate;
 
-      eISR_Rate = (OCR1A_nominal >> 2) * step_loops_nominal / abs(e_steps[TOOL_E_INDEX]);
+      eISR_Rate = (OCR1A_nominal >> 3) * step_loops_nominal / abs(e_steps[TOOL_E_INDEX]);
 
     #endif
 

Marlin/stepper.h

       static unsigned char old_OCR0A;
       static volatile unsigned char eISR_Rate;
       #if ENABLED(LIN_ADVANCE)
-        static volatile long e_steps[E_STEPPERS];
+        static volatile int e_steps[E_STEPPERS];
         static int extruder_advance_k;
         static int final_estep_rate;
         static int current_estep_rate[E_STEPPERS]; // Actual extruder speed [steps/s]