i965: Don't do vector splitting for ir_var_system_value

i965: Don't do vector splitting for ir_var_system_value

This is required while adding builtin system value vec{2, 3, 4}
variables. For example:
(declare (sys) vec2 gl_SamplePosition)

Without this patch above glsl ir splits in to:
(declare (temporary) float gl_SamplePosition_x)
(declare (temporary) float gl_SamplePosition_y)

Signed-off-by: Anuj Phogat <anuj.phogat@gmail.com>
Reviewed-by: Eric Anholt <eric@anholt.net>