[mlir][Vector] Provide progressive lowering of masked n-D vector transfers

This revision allows masked vector transfers with m-D buffers and n-D vectors to
progressively lower to m-D buffer and 1-D vector transfers.

For a vector.transfer_read, assuming a `memref<(leading_dims) x (major_dims) x (minor_dims) x type>` and a `vector<(minor_dims) x type>` are involved in the transfer, this generates pseudo-IR resembling:
```
     if (any_of(%ivs_major + %offsets, <, major_dims)) {
       %v = vector_transfer_read(
         {%offsets_leading, %ivs_major + %offsets_major, %offsets_minor},
          %ivs_minor):
         memref<(leading_dims) x (major_dims) x (minor_dims) x type>,
         vector<(minor_dims) x type>;
     } else {
       %v = splat(vector<(minor_dims) x type>, %fill)
     }
```

Differential Revision: https://reviews.llvm.org/D79062

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