Problem with IR2112 on Proteus
I think what you're seeing there is an aliasing effect between the PWM switching and the sampling rate of the scope. What it should look like is the red trace, still switching from the positive supply to the negative, with only the width changing from the modulation. The blue will be the smoothed voltage applied to the load. 

The IR2110/2 don't have any dead time insertion, so unless your SPWM algorithm is doing that with the complementary signal in code, it is going to cause you problems in real life with shoot-through. Either use a driver with a dead-time control, or one of the PICs with either the motor control PWM generators, or the CWG that can generate the appropriately timed signals.