constrain doesn't modify the variable that you pass in it, instead it returns a new value, so must be used like this:

ledPinVal = constrain( ledPinVal, 0, 255 );

Or, using if statement:

if ( ledPinVal < 0 )
{
  ledPinVal = 0;
}
else if ( ledPinVal > 255 )
{
  ledPinVal = 255;
}

constrain(ledPinVal, 0, 255); doesn't limit the value to 255. It keeps going up >255 and <0 as well.

But this statement

if (ledPinVal <= 0 || ledPinVal >= 255) {
  ledPinVal=255;//works.

Same for

if (ledPinVal <= 0 || ledPinVal >= 255) {
  ledPinVal=0;//works