[CostModel] remove cost-kind predicate for memcpy cost

The default implementation base returns TCC_Expensive (currently
set to '4'), so that explains the test diff. This probably does
not make sense for most callers, but at least now the costs will
be consistently wrong instead of mysteriously wrong.

The ARM target has an override that tries to model codegen expansion,
and that should likely be adapted for general usage.

This probably does not affect anything because the vectorizers are
the primary users of the throughput cost, but memcpy is not listed
as a trivially vectorizable intrinsic.