i965: Generate vs code using scalar backend for BDW+

With everything in place, we can now use the scalar backend compiler for
vertex shaders on BDW+.  We make scalar vertex shaders the default on
BDW+ but add a new vec4vs debug option to force the vec4 backend.

No piglit regressions.

Performance impact is minimal, I see a ~1.5 improvement on the T-Rex
GLBenchmark case, but in general it's in the noise.  Some of our
internal synthetic, vs bounded benchmarks show great improvement, 20%-40%
in some cases, but real-world cases are mostly unaffected.

Signed-off-by: Kristian Høgsberg <krh@bitplanet.net>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>

src/mesa/drivers/dri/i965/intel_debug.c

@@ -68,6 +68,7 @@ static const struct dri_debug_control debug_control[] = {
    { "optimizer",   DEBUG_OPTIMIZER },
    { "ann",         DEBUG_ANNOTATION },
    { "no8",         DEBUG_NO8 },
+   { "vec4vs",      DEBUG_VEC4VS },
    { NULL,    0 }
 };