i965/fs: Make fs_reg::effective_width take fs_inst* instead of fs_visitor*

i965/fs: Make fs_reg::effective_width take fs_inst* instead of fs_visitor*

   Now that we have execution sizes, we can use that instead of the
   dispatch width.  This way it also works for 8-wide instructions in
   SIMD16.

i965/fs: Make effective_width a variable instead of a function

i965/fs: Preserve effective width in constant propagation

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