[mlir][Vector] Initial masking support in Linalg vectorizer

This patch introduces the initial bits to support vector masking
using the `vector.mask` operation. Vectorization changes should be
NFC for non-masked cases. We can't test masked cases directly until
we extend the Transform dialect to support masking.

Reviewed By: nicolasvasilache

Differential Revision: https://reviews.llvm.org/D137690