nv50/ir: optimize mad/fma with third argument 0 to mul

nv50/ir: optimize mad/fma with third argument 0 to mul

Very modest effect, but it's clearly the right thing to do.

total instructions in shared programs : 6131491 -> 6131398 (-0.00%)
total gprs used in shared programs    : 910157 -> 910131 (-0.00%)
total local used in shared programs   : 15328 -> 15328 (0.00%)

                local        gpr       inst      bytes
    helped           0          55          85          85
      hurt           0          26          20          20

Signed-off-by: Karol Herbst <nouveau@karolherbst.de>
Reviewed-by: Ilia Mirkin <imirkin@alum.mit.edu>