buggy dirty mechanism in hls-graph

[soulomoon]

Following situation could trigger a bug

rule runs the first time
rule runs with dep changed, recomputed but result won't change
rule runs with itself marked dirty, the rule did not rerun
rule runs with itself marked dirty, the rule reruns which it should not

Also comment should added to specify how the hls-graph dirty mechanism is working.

[soulomoon]

resultBuilt might updated to an older value, which make a rule result that should be clean back to dirty. It should trigger unwanted recomputation