Planner acceleration bugfix and speedup v2

.) Use already existing inverse_millimeters instead of /
block->millimeters.
.) Prevent overflow during acceleration calculation by checking if float
is necessary. Idea modified from Sailfish.
.) Save two uint32_t or even float multiplications by checking if
step[AXIS] has steps and if max acceleration is lower than accel. If
not, there is no need to check this axis.

Marlin/planner.h

@@ -166,6 +166,11 @@ class Planner {
      * Nominal speed of previous path line segment
      */
     static float previous_nominal_speed;
+	
+	/**
+ 	 * Limit where 64bit math is necessary for acceleration calculation
+ 	 */
+ 	static uint32_t cutoff_long;
 
     #if ENABLED(DISABLE_INACTIVE_EXTRUDER)
       /**