[X86] Remove FMA4 scalar intrinsics. Use llvm.fma intrinsic instead.

The intrinsics can be implemented with a f32/f64 llvm.fma intrinsic and an insert into a zero vector.

There are a couple regressions here due to SelectionDAG not being able to pull an fneg through an extract_vector_elt. I'm not super worried about this though as InstCombine should be able to do it before we get to SelectionDAG.

llvm-svn: 336416