[CostModel] Templatize EntryCost::Cost to allow custom cost metrics

[CostModel] Templatize EntryCost::Cost to allow custom cost metrics

We currently use an unsigned value for our CostTblEntry and TypeConversionCostTblEntry cost tables which is limiting depending on how the target wishes to handle various CostKinds etc.

For instance, targets might wish to store separate instruction count, latency or throughput values etc. On D46276 we have been investigating storing a code snippet to improve latency/throughput cost calculations.

There is a slight problem in that template argument deduction was struggling to match the now templatized Costs[] tables in a ArrayRef constructor - I've added helper wrappers for CostTableLookup/ConvertCostTableLookup which avoids us having to update all existing calls with a template hint.

Differential Revision: https://reviews.llvm.org/D106351