nv50/ir: optimize imul/imad to xmads

nv50/ir: optimize imul/imad to xmads

This hits the shader-db numbers a good bit, though a few xmads is way
faster than an imul or imad and the cost is mitigated by the next commit,
which optimizes many multiplications by immediates into shorter and less
register heavy instructions than the xmads.

total instructions in shared programs : 5768871 -> 5820882 (0.90%)
total gprs used in shared programs    : 669919 -> 670595 (0.10%)
total shared used in shared programs  : 548832 -> 548832 (0.00%)
total local used in shared programs   : 21068 -> 21164 (0.46%)

                local     shared        gpr       inst      bytes
    helped           0           0          38           0           0
      hurt           1           0         365        3076        3076

Signed-off-by: Rhys Perry <pendingchaos02@gmail.com>
Reviewed-by: Karol Herbst <kherbst@redhat.com>