Simplify backlash compensation code. (#12813)

- Use `TEST(dm,axis)` to determine directions instead of doing comparisons.
- Remove recomputation of `millimeters` and `delta_mm` since backlash compensation should not affect the distance over which material is extruded.

Marlin/src/module/planner.cpp

 }
 
 #if ENABLED(S_CURVE_ACCELERATION)
-
   #ifdef __AVR__
     /**
      * This routine returns 0x1000000 / d, getting the inverse as fast as possible.
     #endif
   #endif
 
-  void Planner::add_backlash_correction_steps(const int32_t da, const int32_t db, const int32_t dc, const uint8_t dm, block_t * const block, float (&delta_mm)[ABCE]) {
+  void Planner::add_backlash_correction_steps(const int32_t da, const int32_t db, const int32_t dc, const uint8_t dm, block_t * const block) {
     static uint8_t last_direction_bits;
     uint8_t changed_dir = last_direction_bits ^ dm;
     // Ignore direction change if no steps are taken in that direction
       if (!changed_dir) return;
     #endif
 
-    const bool positive[XYZ] = {  da > 0,  db > 0, dc > 0 };
-    #ifdef BACKLASH_SMOOTHING_MM
-      const bool non_zero[XYZ] = { da != 0, db != 0, dc != 0 };
-    #endif
-    bool made_adjustment = false;
+    LOOP_XYZ(axis) {
+      if (backlash_distance_mm[axis]) {
+        const bool reversing = TEST(dm,axis);
 
-    LOOP_XYZ(i) {
-      if (backlash_distance_mm[i]) {
         // When an axis changes direction, add axis backlash to the residual error
-        if (TEST(changed_dir, i))
-          residual_error[i] += backlash_correction * (positive[i] ? 1.0f : -1.0f) * backlash_distance_mm[i] * planner.settings.axis_steps_per_mm[i];
+        if (TEST(changed_dir, axis))
+          residual_error[axis] += backlash_correction * (reversing ? -1.0f : 1.0f) * backlash_distance_mm[axis] * planner.settings.axis_steps_per_mm[axis];
 
         // Decide how much of the residual error to correct in this segment
-        int32_t error_correction = residual_error[i];
+        int32_t error_correction = residual_error[axis];
         #ifdef BACKLASH_SMOOTHING_MM
           if (error_correction && backlash_smoothing_mm != 0) {
             // Take up a portion of the residual_error in this segment, but only when
             // the current segment travels in the same direction as the correction
-            if (non_zero[i] && positive[i] == (error_correction > 0)) {
+            if (reversing == (error_correction < 0)) {
               if (segment_proportion == 0)
                 segment_proportion = MIN(1.0f, block->millimeters / backlash_smoothing_mm);
               error_correction *= segment_proportion;
         #endif
         // Making a correction reduces the residual error and modifies delta_mm
         if (error_correction) {
-          block->steps[i] += ABS(error_correction);
-          residual_error[i] -= error_correction;
-          delta_mm[i] = (positive[i] ? 1.0f : -1.0f) * block->steps[i] * steps_to_mm[i];
-          made_adjustment = true;
+          block->steps[axis] += ABS(error_correction);
+          residual_error[axis] -= error_correction;
         }
       }
     }
-
-    // If any of the axes were adjusted, recompute block->millimeters
-    if (made_adjustment)
-      block->millimeters = SQRT(sq(delta_mm[X_AXIS]) + sq(delta_mm[Y_AXIS]) + sq(delta_mm[Z_AXIS]));
   }
 #endif // BACKLASH_COMPENSATION
 
         #endif
       );
 
+    /**
+     * At this point at least one of the axes has more steps than
+     * MIN_STEPS_PER_SEGMENT, ensuring the segment won't get dropped as
+     * zero-length. It's important to not apply corrections
+     * to blocks that would get dropped!
+     *
+     * A correction function is permitted to add steps to an axis, it
+     * should *never* remove steps!
+     */
     #if ENABLED(BACKLASH_COMPENSATION)
-      // If we make it here, at least one of the axes has more steps than
-      // MIN_STEPS_PER_SEGMENT, so the segment won't get dropped by Marlin
-      // and it is okay to add steps for backlash correction.
-      add_backlash_correction_steps(da, db, dc, dm, block, delta_mm);
+      add_backlash_correction_steps(da, db, dc, dm, block);
     #endif
   }
 
   float vmax_junction_sqr; // Initial limit on the segment entry velocity (mm/s)^2
 
   #if ENABLED(JUNCTION_DEVIATION)
-
     /**
      * Compute maximum allowable entry speed at junction by centripetal acceleration approximation.
      * Let a circle be tangent to both previous and current path line segments, where the junction

Marlin/src/module/planner.h

     #endif
 
     #if ENABLED(BACKLASH_COMPENSATION)
-      static void add_backlash_correction_steps(const int32_t da, const int32_t db, const int32_t dc, const uint8_t dm, block_t * const block, float (&delta_mm)[ABCE]);
+      static void add_backlash_correction_steps(const int32_t da, const int32_t db, const int32_t dc, const uint8_t dm, block_t * const block);
     #endif
 
   public: