Add a subtype parameter to VTTI::getShuffleCost

In order to cost subvector insertion and extraction, we need to know
the type of the subvector being extracted.

No functionality change.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@171453 91177308-0d34-0410-b5e6-96231b3b80d8