nv50/ir: start LocalCSE with getFirst to merge PHI instructions

nv50/ir: start LocalCSE with getFirst to merge PHI instructions

total instructions in shared programs : 3499888 -> 3499445 (-0.01%)
total gprs used in shared programs    : 453866 -> 453803 (-0.01%)
total local used in shared programs   : 21621 -> 21621 (0.00%)
total bytes used in shared programs   : 32078952 -> 32074936 (-0.01%)

                local        gpr       inst      bytes
    helped           0          39         119         119
      hurt           0           0           0           0

Signed-off-by: Karol Herbst <karolherbst@gmail.com>
Reviewed-by: Ilia Mirkin <imirkin@alum.mit.edu>
Reviewed-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>