i965/vec4: fix assert to detect SIMD lowered DF instructions in IVB

i965/vec4: fix assert to detect SIMD lowered DF instructions in IVB

On IVB, DF instructions have lowered the SIMD width to 4 but the
exec_size will be later doubled. Fix the assert to avoid crashing in
this case.

Signed-off-by: Samuel Iglesias Gonsálvez <siglesias@igalia.com>
[ Francisco Jerez: Simplify assert.  Except for the 'inst->group % 4
  == 0' part the assertion was redundant with the previous assertion. ]
Reviewed-by: Francisco Jerez <currojerez@riseup.net>