[ARM] Attempt to distribute reductions

This adds a combine for adds of reductions, distributing them so that
they occur sequentially to enable better use of accumulating VADDVA
instructions. It combines:
  add(X, add(vecreduce(Y), vecreduce(Z))) ->
    add(add(X, vecreduce(Y)), vecreduce(Z))
and
  add(add(A, reduce(B)), add(C, reduce(D))) ->
    add(add(add(A, C), reduce(B)), reduce(D))

These together distribute the add's so that more reductions can be
selected to VADDVA.

Differential Revision: https://reviews.llvm.org/D106532