[mlir][XeGPU] Use DistributeLayoutAttr instead of LayoutAttr for load gather/scatter ops (#167850)
The PR changes the layout attribute type for
`xegpu::LoadGatherOp/StoreScatterOp` from `LayoutAttr` to
`DistributeLayoutAttr` to also support `xegpu.slice` layouts.
Initially we [wanted to restrict slice
layouts](https://github.com/llvm/llvm-project/pull/163414#discussion_r2478978798)
from the attribute, but now it turns out there are actually valid use
cases for that:
```mlir
gpu.func @distribute_load_slice_attr() {
%2 = memref.alloca() {alignment = 1024} : memref<4096xf32>
%offset = arith.constant {layout_result_0 = #xegpu.layout<sg_layout = [8], sg_data = [32], inst_data = [16]> } dense<0> : vector<256xindex>
%mask = arith.constant {layout_result_0 = #xegpu.layout<sg_layout = [8], sg_data = [32], inst_data = [16]> } dense<1> : vector<256xi1>
%3 = xegpu.load %2[%offset], %mask <{chunk_size = 1, layout = #xegpu.slice<#xegpu.layout<sg_layout = [8, 8], sg_data = [32, 32], inst_data = [8, 16]>, dims = [0]>>} {
layout_result_0 = #xegpu.slice<#xegpu.layout<sg_layout = [8, 8], sg_data = [32, 32], inst_data = [8, 16]>, dims = [0]>
} : memref<4096xf32>, vector<256xindex>, vector<256xi1> -> vector<256xf32>
%4 = vector.broadcast %3 {layout_result_0 =
#xegpu.layout<sg_layout = [8, 8], sg_data = [32, 32], inst_data = [8, 16]>} : vector<256xf32> to vector<256x256xf32>
gpu.return
}
```
Signed-off-by: dchigarev <dmitry.chigarev@intel.com>
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