[InstCombine] reassociate FP diff of sums into sum of diffs

(a[0] + a[1] + a[2] + a[3]) - (b[0] + b[1] + b[2] +b[3]) -->
(a[0] - b[0]) + (a[1] - b[1]) + (a[2] - b[2]) + (a[3] - b[3])

This should be the last step in solving PR43953:
https://bugs.llvm.org/show_bug.cgi?id=43953

We started emitting reduction intrinsics with:
D80867/ rGe50059f6b6b3
So it's a relatively easy pattern match now to re-order those ops.
Also, I have not seen any complaints for the switch to intrinsics
yet, so I'll propose to remove the "experimental" tag from the
intrinsics soon.

Differential Revision: https://reviews.llvm.org/D81491