i965/vec4: don't do horizontal stride on some register file types

i965/vec4: don't do horizontal stride on some register file types

horiz_offset() shouldn't be doing anything for scalar registers,
because all channels of any SIMD instructions will end up reading or
writing the same component of the register, so shifting the register
offset would be wrong.

Signed-off-by: Samuel Iglesias Gonsálvez <siglesias@igalia.com>
[ Francisco Jerez: Re-implement in terms of is_uniform() for
  simplicity.  Pass argument by const reference.  Clarify commit
  message. ]
Reviewed-by: Francisco Jerez <currojerez@riseup.net>