X86: Promote sitofp <8 x i16> to <8 x i32> when AVX is available.

A vector sext + sitofp is a lot cheaper than 8 scalar conversions.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@178448 91177308-0d34-0410-b5e6-96231b3b80d8