nv50/ir/ra: prefer def == src2 for fma with immediates on nvc0

nv50/ir/ra: prefer def == src2 for fma with immediates on nvc0

This helps with the PostRALoadPropagation pass moving long immediates into
FMA/MAD instructions.

changes in shader-db:
total instructions in shared programs : 5894114 -> 5886074 (-0.14%)
total gprs used in shared programs    : 666558 -> 666563 (0.00%)
total shared used in shared programs  : 520416 -> 520416 (0.00%)
total local used in shared programs   : 53524 -> 53524 (0.00%)
total bytes used in shared programs   : 54006744 -> 53932472 (-0.14%)

                local     shared        gpr       inst      bytes
    helped           0           0           2        4192        4192
      hurt           0           0           7           9           9

Signed-off-by: Karol Herbst <karolherbst@gmail.com>
[imirkin: minor edits to separate nv50 and nvc0+ cases]
Reviewed-by: Ilia Mirkin <imirkin@alum.mit.edu>