i965/vec4: Combine all the math emitters.

i965/vec4: Combine all the math emitters.

17 insertions(+), 102 deletions(-).  Works just as well.

v2: Make emit_math take const references (suggested by Matt),
    drop redundant WRITEMASK_XYZW setting (Matt and Curro).

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Matt Turner <mattst88@gmail.com>
Reviewed-by: Francisco Jerez <currojerez@riseup.net>