i965: Only convert if/else to conditional adds prior to Gen6.

i965: Only convert if/else to conditional adds prior to Gen6.

Normally when outputting instructions in SPF (single program flow)
mode, we convert IF and ELSE instructions to conditional ADD
instructions applied to the IP register.  On platforms prior to Gen6,
flow control instructions cause an implied thread switch, so this is a
significant savings.

However, according to the SandyBridge PRM (Volume 4 part 2, p79):

   [Errata DevSNB{WA}] - When SPF is ON, IP may not be updated by
   non-flow control instructions.

So we have to disable this optimization on Gen6.

On later platforms, there is no significant benefit to converting flow
control instructions to ADDs, so for the sake of consistency, this
patch disables the optimization on later platforms too.

The reason we never noticed this problem before is that so far we
haven't needed to use SPF mode on Gen6.  However, later patches in
this series will introduce a Gen6 GS program which uses SPF mode.

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