i965/gen4-5: Program the execution size correctly for DO/WHILE instructions.

From the hardware docs for the DO instruction:

 "Execution size is ignored for this instruction."

My observation on ILK hardware contradicts the spec though, channels
over the execution size of a DO instruction won't enter the loop, and
channels over the execution size of a WHILE instruction will exit the
loop after the first iteration -- The latter is consistent with the
spec though, there's no claim about the execution size being ignored
for the WHILE instruction so it's not completely unexpected that it
has an influence on the evaluation of EMask.

The execute_size argument of brw_DO() shouldn't have any effect on
Gen6 and newer hardware.  On Gen4-5 WHILE instructions inherit the
execution size from the matching DO, so this patch should fix them
too.  The execution size of BREAK and CONT instructions was already
being set correctly.

Fixes some 50 piglit tests on Gen4-5 when forced to run shaders with
conditional and loop instructions 16-wide,
e.g. shaders/glsl-fs-continue-inside-do-while.

Reviewed-by: Matt Turner <mattst88@gmail.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>

src/mesa/drivers/dri/i965/brw_fs_generator.cpp

@@ -1869,7 +1869,7 @@ fs_generator::generate_code(const cfg_t *cfg, int dispatch_width)
 	 break;
 
       case BRW_OPCODE_DO:
-	 brw_DO(p, BRW_EXECUTE_8);
+	 brw_DO(p, dispatch_width == 16 ? BRW_EXECUTE_16 : BRW_EXECUTE_8);
 	 break;
 
       case BRW_OPCODE_BREAK: