[mlir][Vector] Add folding for masked reductions and vector.mask

This patch adds support for folding trivial masked reductions and
multi-reductions (e.g., multi-reductions with only parallel dims,
reductions of a single element, etc.). To support those foldings in
a composable way we also add support for folding different flavors of
empty vector.mask opertions.

Reviewed By: ThomasRaoux

Differential Revision: https://reviews.llvm.org/D144414