nv50/ir: optimize multiplication by 16-bit immediates into two xmads

nv50/ir: optimize multiplication by 16-bit immediates into two xmads

Rather than the usual three that would be created.

total instructions in shared programs : 5796385 -> 5786560 (-0.17%)
total gprs used in shared programs    : 670103 -> 669968 (-0.02%)
total shared used in shared programs  : 548832 -> 548832 (0.00%)
total local used in shared programs   : 21164 -> 21068 (-0.45%)

                local     shared        gpr       inst      bytes
    helped           1           0          64        1040        1040
      hurt           0           0          27           0           0

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