i965/vs: Add a pass to set dependency control fields on instructions.

i965/vs: Add a pass to set dependency control fields on instructions.

This is a more aggressive version of the old brw_optimize() path.  Reduces
cycles spent in the vertex shader on minecraft by 18.6% +/- 10.0% (n=15).

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