solver: simplify edge-related functions in the solver #5116

Job	Run time
prepare	5s
govulncheck	58s
binaries (darwin/amd64)	7m 37s
binaries (darwin/arm64)	3m 21s
binaries (linux/amd64)	4m 6s
binaries (linux/arm/v7)	4m 16s
binaries (linux/arm64)	4m 4s
binaries (linux/s390x)	4m 4s
binaries (linux/ppc64le)	4m 8s
binaries (linux/riscv64)	4m 6s
binaries (windows/amd64)	3m 46s
binaries (windows/arm64)	3m 47s
test / prepare	6m 28s
test / run (containerd, ./client ./cmd/buildctl ./worker/containerd ./solver ./frontend, integration)	14m 54s
test / run (containerd-rootless, ./client ./cmd/buildctl ./worker/containerd ./solver ./frontend, integr...	19m 20s
test / run (containerd-1.6, ./client ./cmd/buildctl ./worker/containerd ./solver ./frontend, integration)	19m 50s
test / run (containerd-snapshotter-stargz, ./client ./cmd/buildctl ./worker/containerd ./solver ./fronte...	20m 45s
test / run (oci, ./client ./cmd/buildctl ./worker/containerd ./solver ./frontend, integration)	12m 58s
test / run (oci-rootless, ./client ./cmd/buildctl ./worker/containerd ./solver ./frontend, integration)	17m 17s
test / run (oci-rootless-slirp4netns-detachnetns, ./client ./cmd/buildctl ./worker/containerd ./solver ....	16m 11s
test / run (oci-snapshotter-stargz, ./client ./cmd/buildctl ./worker/containerd ./solver ./frontend, int...	17m 43s
test / run (./..., 1, integration gateway)	16m 29s
test / run (./client, containerd, nydus, integration)	14m 52s
test / run (./client, oci, nydus, integration)	17m 36s
test / run (./..., nydus, 1, integration)	15m 33s
image	8m 44s
image (rootless)	8m 48s
release	13s
scout	0s
	4h 31m 59s

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