[vectorizer] Add an override for the target instruction cost and use it

[vectorizer] Add an override for the target instruction cost and use it
to stabilize a test that really is trying to test generic behavior and
not a specific target's behavior.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@200215 91177308-0d34-0410-b5e6-96231b3b80d8