nv50/ir: manually optimize multiplication expansion logic

nv50/ir: manually optimize multiplication expansion logic

The conversion of 32-bit integer multiplies into 16-bit ones happens
after the regular optimization loop. However it's fairly common to
multiply by a small integer, rendering some of the expansion pointless.

Firstly, propagate immediates when possible into mul ops, secondly just
remove the ops when they are unnecessary.

Including the change to generate imad immediates, the effect is:

total instructions in shared programs : 6365463 -> 6351898 (-0.21%)
total gprs used in shared programs    : 728684 -> 728684 (0.00%)
total local used in shared programs   : 9904 -> 9904 (0.00%)
total bytes used in shared programs   : 44001576 -> 44036120 (0.08%)

                local        gpr       inst      bytes
    helped           0           0        3288           4
      hurt           0           0           0         842

It's easy for this to hurt bytes since we end up always generating the
8-byte form, while we can't always get rid of the immediate in question.

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