Lowering for `vector.gather`/`vector.scatter` into `xegpu.load`/`xegpu.store`.
High level steps to lower vector.gather/scatter:
```
%0 = vector.gather %source[%off1, %off2, %off3][%indices], %mask,
%pass_thru : memref<8x16x32xf32>, vector<8xindex>, vector<8xi1>, vector<8xf32> into vector<8xf32>
```
1. Compute strides and a memref offset for the `%source` memref using
`computeMemrefMeta` func from the transfer_read/write lowering
2. Compute a linear offset like `%lin_off = %base_offset + %off1 *
strides#0 + %off2 * strides#1 + %off3 * strides#2`
3. Combine the linear offset with `%indices`: `%off = (broadcast
%lin_off : index to vector<8xindex>) + %indices * strides#2`
4. Convert memref to an i64: `%flat_memref =
memref.extract_aligned_pointer_as_index %source + arith.index_cast`
5. Perform load/store: `%vec = xegpu.load %flat_memref[%off], %mask`
6. Apply selection to propagate values from the pass_thru vector: `%res
= arith.select %mask, %vec, %pass_thru`
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