There is an old trick to do this. Just replace the floor material (make it white), render and save the irradiance map(there is an option/button to save it) then, put back the desired material, load the irradiance map that you just saved, and render the final image.
Cheers!
PS: For speed up things, check in "Global switches" under "GI" , "Don't render image". In this way, only GI will be calculated.