[mlir][Vector] Add an optional "masked" boolean array attribute to vector transfer...

[mlir][Vector] Add an optional "masked" boolean array attribute to vector transfer operations

Summary:
Vector transfer ops semantic is extended to allow specifying a per-dimension `masked`
attribute. When the attribute is false on a particular dimension, lowering to LLVM emits
unmasked load and store operations.

Differential Revision: https://reviews.llvm.org/D80098