[TTI] Improve description of TargetCostKind enums to aid targets in choosing cost values

I'm not sure how much to add to the description as we've tried to allow targets to interpret the TargetCostKind enums in their own way. But we need to make it clear that certain cost kinds need to match threshold numbers used by various passes (and vice-versa when passes are determining a cost-benefit threshold).

I'm not keen on the "The weighted sum of size and latency" description, but its very difficult to come up with anything else that's suitably generic (e.g. X86 will use uop counts here to easily work with LoopMicroOpBufferSize thresholds, even though high latency fdiv/fsqrt instructions still often have low uop counts).

Differential Revision: https://reviews.llvm.org/D132288

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