nv50,nvc0: disable the TGSI merge registers pass

nv50,nvc0: disable the TGSI merge registers pass

shader-db results on GK106 (Thanks Karol):

total instructions in shared programs : 3931608 -> 3929463 (-0.05%)
total gprs used in shared programs    : 481255 -> 479014 (-0.47%)
total local used in shared programs   : 27481 -> 27381 (-0.36%)
total bytes used in shared programs   : 36031256 -> 36011120 (-0.06%)

                local        gpr       inst      bytes
    helped          14        1471        1309        1309
      hurt           1          88         384         384

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