nvc0/ir: also do ConstantFolding for FMA

nvc0/ir: also do ConstantFolding for FMA

Helps mainly Feral-ported games, due to their use of fma()

shader-db changes:
total instructions in shared programs : 3941587 -> 3940749 (-0.02%)
total gprs used in shared programs    : 481511 -> 481460 (-0.01%)
total local used in shared programs   : 27469 -> 27481 (0.04%)
total bytes used in shared programs   : 36123344 -> 36115776 (-0.02%)

                local        gpr       inst      bytes
    helped           2          48         243         243
      hurt           2           3          32          32

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