[X86][Costmodel] Fix `X86TTIImpl::getGSScalarCost()`

`X86TTIImpl::getGSScalarCost()` has (at least) two issues:
* it naively computes the cost of sequence of `insertelement`/`extractelement`.
  If we are operating not on the XMM (but YMM/ZMM),
  this widely overestimates the cost of subvector insertions/extractions.
* Gather/scatter takes a vector of pointers, and scalarization results in us performing
  scalar memory operation for each of these pointers, but we never account for the cost
  of extracting these pointers out of the vector of pointers.

Reviewed By: RKSimon

Differential Revision: https://reviews.llvm.org/D111222