i965/vs: Try to emit more components of constant vectors at once.

i965/vs: Try to emit more components of constant vectors at once.

We were naively emitting each component at a time, even if we were
emitting the same value to multiple channels.  Improves on a codegen
regression from the old VS to the new VS on some unigine shaders
(because we emit constant vecs/matrices as immediates instead of
loading them as push constants, so we had over 4x the instructions for
using them).

shader-db results:
Total instructions: 58594 -> 58540
11/870 programs affected (1.3%)
765 -> 711 instructions in affected programs (7.1% reduction)