[InstCombine] reassociate diff of sums into sum of diffs

This is the integer sibling to D81491.

(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])

Removing the "experimental" from these intrinsics is likely
not too far away.