[mlir][Vector] Add support for masked vector.contract

This patch adds support for masking vector.contract ops with the
vector.mask approach. This also includes the lowering of vector.contract
through the vector.outerproduct path to LLVM. For now, this only adds
support for one of the many potential flavors of
vector.contract/vector.outerproduct but unsupported cases will fail
gratefully.

Reviewed By: ThomasRaoux

Differential Revision: https://reviews.llvm.org/D143965