[X86] Lower the cost of v2i32->v2f64 sint_to_fp under vector widening legalization.

[X86] Lower the cost of v2i32->v2f64 sint_to_fp under vector widening legalization.

I don't really understand the costs we're using for fp_to_sint,
but prior to widening legalization we used 20 as the cost for this
via the v2i64->v2f64 entry. That number seems better than the 40
we got with widening legalization. So now we need either a
v2i32->v2f64 entry or a v4i32->v2f64 entry depending on whether
AVX is enabled or not since we skip the first SSE2 table look up
under AVX.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@369628 91177308-0d34-0410-b5e6-96231b3b80d8