[mlir][Vector] Add a masked vectorization of tensor.pad

This revision takes advantage of masking support to introduce a vectorized
version of pad that does not require lowering to lower-level form.

Lowering to lower-level form (if/else + generate + fill + copy + insert_slice)
creates unnecessary complexity that can be completely sidestepped by using
masked vectorization properly.

Differential Revision: https://reviews.llvm.org/D148261