[DAG] Fold Op(vecreduce(a), vecreduce(b)) into vecreduce(Op(a,b))

So long as the operation is reassociative, we can reassociate the double
vecreduce from for example fadd(vecreduce(a), vecreduce(b)) to
vecreduce(fadd(a,b)). This will in general save a few instructions, but some
architectures (MVE) require the opposite fold, so a shouldExpandReduction is
added to account for it. Only targets that use shouldExpandReduction will be
affected.

Differential Revision: https://reviews.llvm.org/D141870