[X86] Move getGatherOverhead/getScatterOverhead into X86TargetTransformInfo.

These cost methods don't make much sense in X86Subtarget. Make
them methods in X86's TTI and move the feature checks from the
X86Subtarget constructor into these methods.

Reviewed By: RKSimon

Differential Revision: https://reviews.llvm.org/D84594