[mlir][async]: Make async.execute operation pure

Author: Aviad Cohen

mlir/include/mlir/Dialect/Async/IR/AsyncOps.td

@@ -38,7 +38,8 @@ def Async_ExecuteOp :
                                                  ["getEntrySuccessorOperands",
                                                   "areTypesCompatible"]>,
                        AttrSizedOperandSegments,
-                       AutomaticAllocationScope]> {
+                       AutomaticAllocationScope,
+                       Pure]> {
   let summary = "Asynchronous execute operation";
   let description = [{
     The `body` region attached to the `async.execute` operation semantically

mlir/test/Dialect/Async/canonicalize.mlir

@@ -0,0 +1,10 @@
+// RUN: mlir-opt %s -split-input-file -canonicalize | FileCheck %s
+
+// CHECK-NOT: async.execute
+
+func.func @empty_execute() {
+  %token = async.execute {
+    async.yield
+  }
+  return
+}