i965/fs: Add an exec_size field to fs_inst

i965/fs: Add an exec_size field to fs_inst

   This will, eventually, allow us to manage execution sizes of
   instructions in a much more natural way from the fs_visitor level.

i965/fs: Explicitly set instruction execute size a couple of places

i965/blorp: Explicitly set instruction execute sizes

   Since blorp is all 16-wide and nothing isn't, in general, very careful
   about register width, we'll just set it all explicitly.

Signed-off-by: Jason Ekstrand <jason.ekstrand@intel.com>
Reviewed-by: Matt Turner <mattst88@gmail.com>