[mlir][Vector] Let vector.multi_reduction reduce down to a scalar.

vector.multi_reduction currently does not allow reducing down to a scalar.
This creates corner cases that are hard to handle during vectorization.
This revision extends the semantics and adds the proper transforms, lowerings and canonicalizations to allow lowering out of vector.multi_reduction to other abstractions all the way to LLVM.

In a future, where we will also allow 0-d vectors, scalars will still be relevant: 0-d vector and scalars are not equivalent on all hardware.

In the process, splice out the implementation patterns related to vector.multi_reduce into a new file.

Reviewed By: pifon2a

Differential Revision: https://reviews.llvm.org/D111442