[MLIR][XeGPU] Fix isEvenlyDistributable API in xegpu (llvm#164907)

Author: nbpatel

mlir/lib/Dialect/XeGPU/IR/XeGPUDialect.cpp

@@ -113,9 +113,12 @@ bool XeGPUDialect::isEvenlyDistributable(llvm::ArrayRef<int64_t> shape,
       if (layout.size() != shape.size())
         return std::nullopt;
       auto ratio = computeShapeRatio(shape, layout);
-      if (!ratio.has_value())
+      if (ratio.has_value()) {
+        newShape = ratio.value();
+      } else if (!rr || !computeShapeRatio(layout, shape).has_value()) {
         return std::nullopt;
-      newShape = ratio.value();
+      }
+      // Round-robin case: continue with original newShape
     }
 
     if (data.size()) {

mlir/test/Dialect/XeGPU/xegpu-wg-to-sg-unify-ops.mlir

@@ -527,4 +527,11 @@ gpu.module @test_distribution {
     %cst_1 = arith.constant {layout_result_0 = #xegpu.layout<sg_layout = [32, 1], sg_data = [1, 16]>} dense<[[0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15]]> : vector<1x16xindex>
     gpu.return
   }
+
+  // CHECK-LABEL: scalar_broadcast
+  gpu.func @scalar_broadcast(%arg0: index) {
+    // CHECK: vector.broadcast {{.*}} : index to vector<1x1x1xindex>
+    %broadcast = vector.broadcast %arg0 {layout_result_0 = #xegpu.layout<sg_layout = [4, 8, 1], sg_data = [1, 1, 1]>} : index to vector<4x1x1xindex>
+    gpu.return
+  }
 }