i965/vec4: Emit MADs from (x + -(y * z)).

i965/vec4: Emit MADs from (x + -(y * z)).

Same as commit c4fab711 to the fs backend.

total instructions in shared programs: 5945998 -> 5945788 (-0.00%)
instructions in affected programs:     74665 -> 74455 (-0.28%)
helped:                                399
HURT:                                  180

It hurts some programs because we make no attempts in the vec4 backend
to avoid MADs if they have constant (or vector uniform) arguments.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>