i965: Add writes_accumulator flag

i965: Add writes_accumulator flag

Our hardware has an "accumulator" register, which can be used to store
intermediate results across multiple instructions.  Many instructions
can implicitly write a value to the accumulator in addition to their
normal destination register.  This is enabled by the "AccWrEn" flag.

This patch introduces a new flag, inst->writes_accumulator, which
allows us to express the AccWrEn notion in the IR.  It also creates a
n ALU2_ACC macro to easily define emitters for instructions that
implicitly write the accumulator.

Previously, we only supported implicit accumulator writes from the
ADDC, SUBB, and MACH instructions.  We always enabled them on those
instructions, and left them disabled for other instructions.

To take advantage of the MAC (multiply-accumulate) instruction, we
need to be able to set AccWrEn on other types of instructions.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Matt Turner <mattst88@gmail.com>
Signed-off-by: Juha-Pekka Heikkila <juhapekka.heikkila@gmail.com>