llbsolver: add input validation to policy recompute #4943

Job	Run time
build (windows/amd64)	2m 8s
build (freebsd/amd64)	1m 48s
test-freebsd-amd64	3m 22s
test-windows-amd64 (containerd, ./client)	3m 54s
test-windows-amd64 (containerd, ./cmd/buildctl)	2m 41s
test-windows-amd64 (containerd, ./worker/containerd)	1m 37s
test-windows-amd64 (containerd, ./solver)	2m 31s
test-windows-amd64 (containerd, ./frontend)	2m 34s
test-windows-amd64 (containerd, ./frontend/dockerfile)	29m 25s
test-windows-amd64 (./..., 1)	12m 10s
	1h 2m 10s

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