nv50/ir: get rid of memory stores with nop values

nv50/ir: get rid of memory stores with nop values

This happens especially with exports and varying packing, where the last
bits aren't always filled in. We end up trying to do quad-wide stores,
which ends up being a lot of register moves that carefully preserve the
nop value. Instead don't do the stores.

total instructions in shared programs : 6131375 -> 6125267 (-0.10%)
total gprs used in shared programs    : 910139 -> 895501 (-1.61%)
total local used in shared programs   : 15328 -> 15328 (0.00%)

                local        gpr       inst
    helped           0        7442        4693
      hurt           0          90        2687

Most of the helped/hurt instruction changes are by one or two ops
because can no longer do quad-wide stores in all cases.

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>