i965/fs: Don't set exec_all on instructions wider than the original in lower_simd_width.

i965/fs: Don't set exec_all on instructions wider than the original in lower_simd_width.

This could have led to somewhat increased bandwidth usage for lowered
texturing instructions on Gen4 (which is the only case in which
lower_width may be greater than inst->exec_size).  After the previous
patches the invariant mentioned in the comment should no longer be
assumed by any of the other optimization and lowering passes, so the
exec_all() call shouldn't be necessary anymore.

Reviewed-by: Jason Ekstrand <jason.ekstrand@intel.com>