i965: Split Gen4-5 and Gen6+ MATH instruction emitters.

i965: Split Gen4-5 and Gen6+ MATH instruction emitters.

Our existing functions, brw_math and brw_math2, had unclear roles:

Gen4-5 used brw_math for both unary and binary math functions; it never
used brw_math2.  Since operands are already in message registers, this
is reasonable.

Gen6+ used brw_math for unary math functions, and brw_math2 for binary
math functions, duplicating a lot of code.  The only real difference was
that brw_math used brw_null_reg() for src1.

This patch improves brw_math2's assertions to allow both unary and
binary operations, renames it to gen6_math(), and drops the Gen6+ code
out of brw_math().

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