[MLIR][Vector] Enable masked vectorizaton of contraction ops

This patch enables the vectorization of contraction ops using vector
masking. Support for vectorizing contractions is already there so this
is just adding contraction ops to the list of supported ops in
`vectorizeDynamicLinalgOpPrecondition` and adding a test.

Reviewed By: hanchung, awarzynski

Differential Revision: https://reviews.llvm.org/D148865