[X86] Add more extract subvector cost model tests for smaller element sizes and small...

[X86] Add more extract subvector cost model tests for smaller element sizes and smaller than 128-bit vectors.

With the switch to widening legalization, we need to a better
job of costing extractions of less than 128-bits.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@368081 91177308-0d34-0410-b5e6-96231b3b80d8