i965 gen6+: Use 1-wide null operands for IF instructions

i965 gen6+: Use 1-wide null operands for IF instructions

The Sandy Bridge PRM, volume 4, part 2, section 5.3.10 ("5.3.10
Register Region Restrictions") contains the following restriction on
the execution size and operand width of instructions:

   "3. ExecSize must be equal to or greater than Width."

When emitting an IF instruction in single program flow mode on Gen6+,
we use an ExecSize of 1, therefore the Width of each operand must also
be 1.

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