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 <[email protected]>
Reviewed-by: Kenneth Graunke <[email protected]>

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