i965/vec4: Let dead code eliminate trim dead channels.

i965/vec4: Let dead code eliminate trim dead channels.

That is, modify

   mad dst, a, b, c

to be

   mad dst.xyz, a, b, c

if dst.w is never read.

total instructions in shared programs: 811869 -> 805582 (-0.77%)
instructions in affected programs:     168287 -> 162000 (-3.74%)

Reviewed-by: Eric Anholt <eric@anholt.net>