glsl/lower_vector_derefs: Don't use a temporary for TCS outputs

Tessellation control shader outputs act as if they have memory backing
them and you can have multiple writes to different components of the
same vector in-flight at the same time.  When this happens, the load vec
store pattern that gets used by ir_triop_vector_insert doesn't yield the
correct results.  Instead, just emit a sequence of conditional
assignments.

Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Cc: mesa-stable@lists.freedesktop.org
(cherry picked from commit bd17bdc5)