aco: use small_vec in RegCounterMap

This seems to be a little faster.

insert_NOPs (navi31):

Difference at 95.0% confidence
	-11.484 +/- 6.13377
	-1.62767% +/- 0.860593%
	(Student's t, pooled s = 5.71913)

insert_NOPs (gfx1200):

Difference at 95.0% confidence
	-35.6745 +/- 4.97972
	-8.1236% +/- 1.10453%
	(Student's t, pooled s = 4.6431)