[X86][Vectorizer Cost Model] Correct vectorization cost model for v2i64->v2f64
and v4i64->v4f64.

The new costs match what we did for SSE2 and reflect the reality of our codegen.

<rdar://problem/16381225>


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@204884 91177308-0d34-0410-b5e6-96231b3b80d8