Improve the newSpark dud test by using the pointer tag bits
newSpark() checks if the spark is a dud, and if so does not add it to the spark pool. Previously, newSpark would discard the pointer tag bits and just check closure_SHOULD_SPARK(p). We can take advantage of the tag bits which can tell us if the pointer points to a value. If it is, it's a dud spark and we don't need to add it to the spark pool.
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