solver: prevent edge merge to inactive states #3337

Job	Run time
build (windows/amd64)	1m 41s
build (freebsd/amd64)	1m 34s
test-freebsd-amd64	17m 2s
test-windows-amd64 (containerd, ./client)	4m 44s
test-windows-amd64 (containerd, ./cmd/buildctl)	2m 57s
test-windows-amd64 (containerd, ./worker/containerd)	3m 12s
test-windows-amd64 (containerd, ./solver)	3m 21s
test-windows-amd64 (containerd, ./frontend)	3m 26s
test-windows-amd64 (containerd, ./frontend/dockerfile)	5m 39s
test-windows-amd64 (./..., 1)	13m 36s
	57m 12s

Provide feedback