[SLP]Workaround for InsertSubVector cost.

The cost of the InsertSubvector shuffle kind cost is not complete and
may end up with just extracts + inserts costs in many cases. Added
a workaround to represent it as a generic PermuteSingleSrc, which is
still pessimistic but better than InsertSubvector.

Differential Revision: https://reviews.llvm.org/D105827