[VectorCombine] Pull out TargetCostKind argument to allow globally set cost kind value

Some prep work to allow a future patch to potentially use VectorCombine to target code size for -Os/Oz builds (setting TCK_CodeSize instead of TCK_RecipThroughput).

There's still more cleanup to do as a lot of get*Cost calls are relying on the default TargetCostKind value (usually TCK_RecipThroughput but not always).

Author: RKSimon