soft_pwm: avoid useless refetches of pwm_count

The compiler is not able to reuse the value of pwm_count, but reloads it
on every evaluation, if is stored in a static variable, as it cannot prove
it will be unchanged. A variable with local scope may not be modified from
the outside, so its value can be reused.
Doing so reduces text size and instruction count.

Signed-off-by: Stefan Brüns <stefan.bruens@rwth-aachen.de>

Marlin/temperature.cpp

   static uint8_t temp_count = 0;
   static TempState temp_state = StartupDelay;
   static uint8_t pwm_count = _BV(SOFT_PWM_SCALE);
+  // avoid multiple loads of pwm_count
+  uint8_t pwm_count_tmp = pwm_count;
 
   // Static members for each heater
   #if ENABLED(SLOW_PWM_HEATERS)
     /**
      * Standard PWM modulation
      */
-    if (pwm_count >= 127) {
-      pwm_count -= 127;
+    if (pwm_count_tmp >= 127) {
+      pwm_count_tmp -= 127;
       soft_pwm_0 = (soft_pwm_0 & pwm_mask) + soft_pwm[0];
       WRITE_HEATER_0(soft_pwm_0 > pwm_mask ? HIGH : LOW);
       #if HOTENDS > 1
       #endif
     }
 
-    if (soft_pwm_0 <= pwm_count) WRITE_HEATER_0(0);
+    if (soft_pwm_0 <= pwm_count_tmp) WRITE_HEATER_0(0);
     #if HOTENDS > 1
-      if (soft_pwm_1 <= pwm_count) WRITE_HEATER_1(0);
+      if (soft_pwm_1 <= pwm_count_tmp) WRITE_HEATER_1(0);
       #if HOTENDS > 2
-        if (soft_pwm_2 <= pwm_count) WRITE_HEATER_2(0);
+        if (soft_pwm_2 <= pwm_count_tmp) WRITE_HEATER_2(0);
         #if HOTENDS > 3
-          if (soft_pwm_3 <= pwm_count) WRITE_HEATER_3(0);
+          if (soft_pwm_3 <= pwm_count_tmp) WRITE_HEATER_3(0);
         #endif
       #endif
     #endif
 
     #if HAS_HEATER_BED
-      if (soft_pwm_BED <= pwm_count) WRITE_HEATER_BED(0);
+      if (soft_pwm_BED <= pwm_count_tmp) WRITE_HEATER_BED(0);
     #endif
 
     #if ENABLED(FAN_SOFT_PWM)
       #if HAS_FAN0
-        if (soft_pwm_fan[0] <= pwm_count) WRITE_FAN(0);
+        if (soft_pwm_fan[0] <= pwm_count_tmp) WRITE_FAN(0);
       #endif
       #if HAS_FAN1
-        if (soft_pwm_fan[1] <= pwm_count) WRITE_FAN1(0);
+        if (soft_pwm_fan[1] <= pwm_count_tmp) WRITE_FAN1(0);
       #endif
       #if HAS_FAN2
-        if (soft_pwm_fan[2] <= pwm_count) WRITE_FAN2(0);
+        if (soft_pwm_fan[2] <= pwm_count_tmp) WRITE_FAN2(0);
       #endif
     #endif
 
     // 3:                / 16 =  61.0352 Hz
     // 4:                /  8 = 122.0703 Hz
     // 5:                /  4 = 244.1406 Hz
-    pwm_count += _BV(SOFT_PWM_SCALE);
+    pwm_count = pwm_count_tmp + _BV(SOFT_PWM_SCALE);
 
   #else // SLOW_PWM_HEATERS
 
     #endif
 
     #if ENABLED(FAN_SOFT_PWM)
-      if (pwm_count >= 127) {
-        pwm_count = 0;
+      if (pwm_count_tmp >= 127) {
+        pwm_count_tmp = 0;
         #if HAS_FAN0
           soft_pwm_fan[0] = fanSpeedSoftPwm[0] >> 1;
           WRITE_FAN(soft_pwm_fan[0] > 0 ? HIGH : LOW);
         #endif
       }
       #if HAS_FAN0
-        if (soft_pwm_fan[0] <= pwm_count) WRITE_FAN(0);
+        if (soft_pwm_fan[0] <= pwm_count_tmp) WRITE_FAN(0);
       #endif
       #if HAS_FAN1
-        if (soft_pwm_fan[1] <= pwm_count) WRITE_FAN1(0);
+        if (soft_pwm_fan[1] <= pwm_count_tmp) WRITE_FAN1(0);
       #endif
       #if HAS_FAN2
-        if (soft_pwm_fan[2] <= pwm_count) WRITE_FAN2(0);
+        if (soft_pwm_fan[2] <= pwm_count_tmp) WRITE_FAN2(0);
       #endif
     #endif //FAN_SOFT_PWM
 
     // 3:                / 16 =  61.0352 Hz
     // 4:                /  8 = 122.0703 Hz
     // 5:                /  4 = 244.1406 Hz
-    pwm_count += _BV(SOFT_PWM_SCALE);
+    pwm_count = pwm_count_tmp + _BV(SOFT_PWM_SCALE);
 
     // increment slow_pwm_count only every 64th pwm_count,
     // i.e. yielding a PWM frequency of 16/128 Hz (8s).