nir/instr_set: Use _mesa_set_search_or_add()

nir/instr_set: Use _mesa_set_search_or_add()

Before this change, we were searching for each instruction twice, once
when checking if it exists and once when figuring out where to insert
it. By using the new function, we can do everything we need to do in one
operation.

Compilation time numbers for my shader-db database:

Difference at 95.0% confidence
	-4.04706 +/- 0.669508
	-0.922142% +/- 0.151948%
	(Student's t, pooled s = 0.95824)

Reviewed-by: Eric Anholt <eric@anholt.net>
Acked-by: Jason Ekstrand <jason@jlekstrand.net>