[X86][CostModel] X86TTIImpl::getShuffleCost(): subvector insertions are cheap

This is similar to the subvector extractions,
except that the 0'th subvector isn't free to insert,
because we generally don't know whether or not
the upper elements need to be preserved:
https://godbolt.org/z/rsxP5W4sW

This is needed to avoid regressions in D100684

Reviewed By: RKSimon

Differential Revision: https://reviews.llvm.org/D100698