[mlir][Shape] Mark BroadcastOp as not having side effects

[bkramer]

Summary

This allows it to be dead code eliminated when unused.

mlir/include/mlir/Dialect/Shape/IR/ShapeOps.td

def Shape_AddOp : Shape_Op<"add", [Commutative, NoSideEffect]> {
 
   let assemblyFormat = [{
     $lhs `,` $rhs attr-dict `:` type($lhs) `,` type($rhs) `->` type($result)
   }];
 
   let verifier = [{ return verifySizeOrIndexOp(*this); }];
 }
 
-def Shape_BroadcastOp : Shape_Op<"broadcast", [Commutative]> {
+def Shape_BroadcastOp : Shape_Op<"broadcast", [Commutative, NoSideEffect]> {
   let summary = "Returns the broadcasted output shape of two or more inputs";
   let description = [{
     Returns the broadcasted shape for input shapes or extent tensors. The rest
     of this description is simplified for the 2 input case but can be extended
     to more inputs. Both operands can be of type `shape.shape` or
     `tensor<?xindex>`. The result is of type `shape.shape` and, if both
     operands are tensors, may be of type `tensor<?xindex>`.
 

mlir/test/Dialect/Shape/canonicalize.mlir

func @f() -> !shape.shape {
   %0 = shape.const_shape [2] : !shape.shape
   %1 = shape.const_shape [7] : !shape.shape
   %2 = shape.broadcast %0, %1 : !shape.shape, !shape.shape -> !shape.shape
   return %2 : !shape.shape
 }
 
 // -----
 
+// Dead code
+// CHECK-LABEL: @broadcast
+func @broadcast(%arg0 : !shape.shape, %arg1 : !shape.shape) {
+  // CHECK-NEXT: return
+  %0 = shape.broadcast %arg0, %arg1
+      : !shape.shape, !shape.shape -> !shape.shape
+  return
+}
+
+// -----
+
 // Basic case.
 // CHECK-LABEL: func @f
 func @f() -> !shape.shape {
   // CHECK: shape.const_shape [0, 1, 2, 3] : !shape.shape
   %lhs = shape.const_shape [0, 1] : !shape.shape
   %rhs = shape.const_shape [2, 3] : !shape.shape
   %0 = shape.concat %lhs, %rhs
   return %0 : !shape.shape