nv50/ir: flip shl(add, imm) into add(shl, imm)

nv50/ir: flip shl(add, imm) into add(shl, imm)

This works when the add also has an immediate. This often happens in
address calculations. These addresses can then be inlined as well.

On code targeted to SM35:

total instructions in shared programs : 6223346 -> 6206257 (-0.27%)
total gprs used in shared programs    : 911075 -> 911045 (-0.00%)
total local used in shared programs   : 39072 -> 39072 (0.00%)

                local        gpr       inst      bytes
    helped           0         119        3664        3664
      hurt           0          74          15          15

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