[x86] Generalize the single-element insertion lowering to work with
floating point types and use it for both v2f64 and v2i64 single-element
insertion lowering.

This fixes the last non-AVX performance regression test case I've gotten
of for the new vector shuffle lowering. There is obvious analogous
lowering for v4f32 that I'll add in a follow-up patch (because with
INSERTPS, v4f32 requires special treatment). After that, its AVX stuff.