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.

Piglit test: piglit!11 (merged)