ORCIÈRES-MERLETTE, France—As it powers through the towns in the valley, the Tour de France peloton can see the Alpe d’Huez rise above the road long before it reaches the first slopes. The riders can pick out the 21 switchbacks to the top and know exactly what’s coming.  The Alpe is more than a test of their legs or their lungs. With half a million fans waiting along every inch of the road and the French summer sun beating down, the riders brace for the wave of noise and smoke and humanity about to crash down on them for 8.5 miles…