[LSR][term-fold] Adjust expansion budget based on trip count (#80304)

Follow up to https://github.com/llvm/llvm-project/pull/74747



This change extends the previously added fixed expansion threshold by
scaling down the cost allowed for an expansion for a loop with either a
small known trip count or a profile which indicates the trip count is
likely small. The goal here is to improve code generation for a loop
nest where the outer loop has a high trip count, and the inner loop runs
only a handful of iterations.

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Co-authored-by: Nikita Popov <github@npopov.com>