[mlir][arith] Refine the verifier for arith.constant (#86178)

Disallows initialization of scalable vectors with an attribute of
arbitrary values, e.g.:
```mlir
  %c = arith.constant dense<[0, 1]> : vector<[2] x i32>
```

Initialization using vector splats remains allowed (i.e. when all the
init values are identical):
```mlir
  %c = arith.constant dense<[1, 1]> : vector<[2] x i32>
```

Author: banach-space

mlir/lib/Dialect/Arith/IR/ArithOps.cpp

@@ -213,6 +213,12 @@ LogicalResult arith::ConstantOp::verify() {
     return emitOpError(
         "value must be an integer, float, or elements attribute");
   }
+
+  auto vecType = dyn_cast<VectorType>(type);
+  if (vecType && vecType.isScalable() && !isa<SplatElementsAttr>(getValue()))
+    return emitOpError(
+        "intializing scalable vectors with elements attribute is not supported"
+        " unless it's a vector splat");
   return success();
 }
 

mlir/test/Dialect/Arith/invalid.mlir

@@ -64,6 +64,15 @@ func.func @constant_out_of_range() {
 
 // -----
 
+func.func @constant_invalid_scalable_vec_initialization() {
+^bb0:
+  // expected-error@+1 {{'arith.constant' op intializing scalable vectors with elements attribute is not supported unless it's a vector splat}}
+  %c = arith.constant dense<[0, 1]> : vector<[2] x i32>
+  return
+}
+
+// -----
+
 func.func @constant_wrong_type() {
 ^bb:
   %x = "arith.constant"(){value = 10.} : () -> f32 // expected-error {{'arith.constant' op failed to verify that all of {value, result} have same type}}