[CostModel] Unify getMemoryOpCost

Use getMemoryOpCost from the generic implementation of getUserCost
and have getInstructionThroughput return the result of that for loads
and stores.

This also means that the X86 implementation of getUserCost can be
removed with the functionality folded into its getMemoryOpCost.

Differential Revision: https://reviews.llvm.org/D80984