[x86] lower extracted fadd/fsub to horizontal vector math

This would show up if we fix horizontal reductions to narrow as they go along, 
but it's an improvement for size and/or Jaguar (fast-hops) independent of that.

We need to do this late to not interfere with other pattern matching of larger 
horizontal sequences.

We can extend this to integer ops in a follow-up patch.

Differential Revision: https://reviews.llvm.org/D56011

llvm-svn: 350369