i965/vec4: Combine generate_math[12]_gen6 methods.

i965/vec4: Combine generate_math[12]_gen6 methods.

These are trivial to combine: we should just avoid checking the second
operand if it's brw_null_reg.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Matt Turner <mattst88@gmail.com>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>