[lsr][term-fold] Restrict transform to low cost expansions (#74747)

This is a follow up to an item I noted in my submission comment for
e947f95. I don't have a real world example where this is triggering
unprofitably, but avoiding the transform when we estimate the loop to be
short running from profiling seems quite reasonable. It's also now come
up as a possibility in a regression twice in two days, so I'd like to
get this in to close out the possibility if nothing else.

The original review dropped the threshold for short trip count loops. I
will return to that in a separate review if this lands.

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