[mlir][Vector] Re-define masking semantics in vector.transfer ops

Masking hasn't been widely used in vector transfer ops and the semantics
of the mask operand were a bit loose. This patch states that the mask
operand in a vector transfer op is applied to the read/write part of the
operation and, therefore, its shape should match the shape of the
elements read/written from/into the memref/tensor regardless of any
permutation/broadcasting also applied by the transfer operation.

Reviewers: nicolasvasilache

Differential Revision: https://reviews.llvm.org/D138079