[mlir] Fix ConstantOp verifier

This restricts the attributes to integers for constants of type
IndexType. So far an attribute like StringAttr as in

  %c1 = constant "" : index

is valid.

Reviewed By: mehdi_amini

Differential Revision: https://reviews.llvm.org/D98216

mlir/lib/Dialect/StandardOps/IR/Ops.cpp

@@ -1205,10 +1205,9 @@ static LogicalResult verify(ConstantOp &op) {
     return op.emitOpError() << "requires attribute's type (" << value.getType()
                             << ") to match op's return type (" << type << ")";
 
-  if (type.isa<IndexType>() || value.isa<BoolAttr>())
-    return success();
-
   if (auto intAttr = value.dyn_cast<IntegerAttr>()) {
+    if (type.isa<IndexType>() || value.isa<BoolAttr>())
+      return success();
     IntegerType intType = type.cast<IntegerType>();
     if (!intType.isSignless())
       return op.emitOpError("requires integer result types to be signless");

mlir/test/Dialect/Standard/invalid.mlir

@@ -247,3 +247,11 @@ func @non_signless_constant() {
   %0 = constant 0 : si32
   return
 }
+
+// -----
+
+func @unsupported_attribute() {
+  // expected-error @+1 {{unsupported 'value' attribute: "" : index}}
+  %0 = constant "" : index
+  return
+}