nv50/ir: optimize shl + and

nv50/ir: optimize shl + and

Address loading can often end up as shl + shr + shl combinations. The
latter two are equal shifts, which get converted into an and mask.
However if the previous shl is more than the mask is trying to remove
(in terms of low bits), we can just remove the and entirely. This
reduces some large shaders by as many as 3% of instructions (out of 2K).

total instructions in shared programs : 6495509 -> 6491076 (-0.07%)
total gprs used in shared programs    : 954621 -> 954623 (0.00%)

                local        gpr       inst      bytes
    helped           0           0        1014        1014
      hurt           0           2           0           0

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