[mlir][arith] Refine the verifier for arith.constant

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

@@ -190,6 +190,14 @@ LogicalResult arith::ConstantOp::verify() {
     return emitOpError(
         "value must be an integer, float, or elements attribute");
   }
+
+  // Intializing scalable vectors with elements attribute is not supported
+  // unless it's a vector splot.
+  auto vecType = dyn_cast<VectorType>(type);
+  auto val = dyn_cast<DenseElementsAttr>(getValue());
+  if ((vecType && val) && vecType.isScalable() && !val.isSplat())
+    return emitOpError(
+        "using elements attribute to initialize a scalable vector");
   return success();
 }
 

mlir/test/Dialect/Arith/invalid.mlir

@@ -215,6 +215,14 @@ func.func @func_with_ops() {
 
 // -----
 
+func.func @func_with_ops() {
+^bb0:
+  // expected-error@+1 {{op failed to verify that result type has i1 element type and same shape as operands}}
+  %c = arith.constant dense<[0, 1]> : vector<[2] x i32>
+}
+
+// -----
+
 func.func @invalid_cmp_shape(%idx : () -> ()) {
   // expected-error@+1 {{'lhs' must be signless-integer-like, but got '() -> ()'}}
   %cmp = arith.cmpi eq, %idx, %idx : () -> ()