[mlir][shape] Add assemblyFormat for `shape.add`.

[herhut]

Neat, thanks!

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

def Shape_AddOp : Shape_Op<"add", [SameOperandsAndResultType]> {
   let description = [{
     Adds two valid sizes as follows:
     * lhs + rhs = unknown if either lhs or rhs unknown;
     * lhs + rhs = (int)lhs + (int)rhs if known;
   }];
 
   let arguments = (ins Shape_SizeType:$lhs, Shape_SizeType:$rhs);
   let results = (outs Shape_SizeType:$result);
+
+  let assemblyFormat = "$lhs `,` $rhs attr-dict";
 }
 
 def Shape_BroadcastOp : Shape_Op<"broadcast", []> {
   let summary = "Returns the broadcasted output shape of two inputs";
   let description = [{
     Computes the broadcasted output shape following:
     1. If any inputs are unranked, output is unranked;
     2. Else the input array with number of dimensions smaller than the max

mlir/test/Dialect/Shape/ops.mlir

 // RUN: mlir-opt -split-input-file %s | mlir-opt | FileCheck %s
 // Verify the printed output can be parsed.
 // RUN: mlir-opt %s | mlir-opt | FileCheck %s
 // Verify the generic form can be parsed.
 // RUN: mlir-opt -mlir-print-op-generic %s | mlir-opt | FileCheck %s
 
 // CHECK-LABEL: shape_num_elements
 func @shape_num_elements(%shape : !shape.shape) -> !shape.size {
   %init = shape.const_size 0
   %num_elements = shape.reduce(%shape, %init) -> !shape.size {
     ^bb0(%index: index, %dim: !shape.size, %lci: !shape.size):
-      %acc = "shape.add"(%lci, %dim) : (!shape.size, !shape.size) -> !shape.size
+      %acc = shape.add %lci, %dim
       shape.yield %acc : !shape.size
   }
   return %num_elements : !shape.size
 }
 
 func @test_shape_num_elements_unknown() {
   %0 = "shape.unknown_shape"() : () -> !shape.shape
   %1 = call @shape_num_elements(%0) : (!shape.shape) -> (!shape.size)