nv50/ir: teach insnCanLoad() about SHLADD

nv50/ir: teach insnCanLoad() about SHLADD

Commutativity is not allowed with SHLADD, but src2 can accept
loads. To allow the load propagation pass to do its job, add a
special case like for SUCLAMP because src1 is always an immediate.

This IMAD to SHLADD optimization helps a bunch of shaders from Tomb
Raider, Victor Vran, UE4 demos (+15% perf with Elemental) and Shadow
Warrior.

GF100/GK104:

total instructions in shared programs :2838045 -> 2834712 (-0.12%)
total gprs used in shared programs    :396684 -> 396386 (-0.08%)
total local used in shared programs   :34416 -> 34416 (0.00%)

                local        gpr       inst      bytes
    helped           0         326        1105        1105
      hurt           0          55           3           3

Signed-off-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Reviewed-by: Ilia Mirkin <imirkin@alum.mit.edu>