[mlir][vector] Add pattern to break down reductions into arith ops (#75727)

The number of vector elements considered 'small' enough to extract is
parameterized.                                                   
                                                                 
This is to avoid going into specialized reduction lowering when a
single/couple of arith ops can do. Targets without dedicated reduction  
intrinsics can use that as an emulation path too.                  
                                                                   
Depends on https://github.com/llvm/llvm-project/pull/75846.