nir_to_tgsi: Store directly to TGSI outputs when possible.

Saves emitting a MOV at the end of the program to store the output.

softpipe glmark2 -b buffer +9.73451% +/- 3.17924% (n=6)
softpipe glmark2 -b build +5.57621% +/- 1.35074% (n=9)```