nv50/ir: optimize sub(a, 0) to a

helped some ue4 demos and divinity OS shaders

total instructions in shared programs : 2818674 -> 2818606 (-0.00%)
total gprs used in shared programs    : 379273 -> 379273 (0.00%)
total local used in shared programs   : 9505 -> 9505 (0.00%)
total bytes used in shared programs   : 25837792 -> 25837192 (-0.00%)

                local        gpr       inst      bytes
    helped           0           0          33          33
      hurt           0           0           0           0

Signed-off-by: Karol Herbst <karolherbst@gmail.com>
Reviewed-by: Ilia Mirkin <imirkin@alum.mit.edu>
Reviewed-by: Pierre Moreau <pierre.morrow@free.fr>