[Target][CodeGen] Remove default CostKind arguments on inner/impl TTI overrides

Based off a discussion on D110100, we should be avoiding default CostKinds whenever possible.

This initial patch removes them from the 'inner' target implementation callbacks - these should only be used by the main TTI calls, so this should guarantee that we don't cause changes in CostKind by missing it in an inner call. This exposed a few missing arguments in getGEPCost and reduction cost calls that I've cleaned up.

Differential Revision: https://reviews.llvm.org/D110242