[x86] lower extracted fadd/fsub to horizontal vector math; 2nd try

The 1st try for this was at rL350369, but it caused IR-level diffs because
our cost models differentiate custom vs. legal/promote lowering. So that was
reverted at rL350373. The cost models were fixed independently at rL350403,
so this is effectively the same patch as last time.

Original commit message:
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: 350421

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