[ARM] Turn vecreduce_add(add(x, y)) into vecreduce(x) + vecreduce(y)

Under MVE we can use VADDV/VADDVA's to perform integer add reductions,
so it can be beneficial to use more reductions than summing subvectors
and reducing once. Especially for VMLAV/VMLAVA the mul can be
incorporated into the reduction, producing less instructions.

Some of the test cases currently get larger due to extra integer adds,
but will be improved in a followup patch.

Differential Revision: https://reviews.llvm.org/D106531