[LLVM][CodeGen][SVE] Add lowering for ISD::VECREDUCE_MUL/FMUL. (#161842)
We might be able to do better by using SVE2 and perhaps even NEON for
the final stages, but this version works everywhere so seems like is a
good place to start.
Fixes https://github.com/llvm/llvm-project/issues/155468