while loop body has a control barrier (`%barrier`) at the end which forces
a dependency and the two assign variable ops must wait for each other to
complete before starting the next iteration. Transforming these control
outputs to data outputs removes the dependency between the two assign
variable ops, thus allowing them to run in parallel across iterations.

Before:

```mlir

name: 'Auto Assign PR to Author'
on:
  pull_request:
    types: [opened]

permissions: {}

jobs:
  add-reviews:
    permissions:
      contents: read  # for kentaro-m/auto-assign-action to fetch config file
      pull-requests: write  # for kentaro-m/auto-assign-action to assign PR reviewers
    runs-on: ubuntu-latest
    steps:

Robert Griesemer <******@****.***> 1607975635 -0800

		j -= 4
		sinSign, cosSign = !sinSign, !cosSign
	}
	if j > 1 {
		cosSign = !cosSign
	}

	zz := z * z
	cos = 1.0 - 0.5*zz + zz*zz*((((((_cos[0]*zz)+_cos[1])*zz+_cos[2])*zz+_cos[3])*zz+_cos[4])*zz+_cos[5])
	sin = z + z*zz*((((((_sin[0]*zz)+_sin[1])*zz+_sin[2])*zz+_sin[3])*zz+_sin[4])*zz+_sin[5])
	if j == 1 || j == 2 {
		sin, cos = cos, sin
	}
	if cosSign {
		cos = -cos
	}
	if sinSign {

	true = false // ERROR "cannot assign to .* (\(declared const\))?"

	var m map[int]struct{ n int }
	m[0].n = 7 // ERROR "cannot assign to struct field .* in map$"

	1 = 7         // ERROR "cannot assign to 1"
	"hi" = 7      // ERROR `cannot assign to "hi"`
	nil = 7       // ERROR "cannot assign to nil"
	len("") = 7   // ERROR `cannot assign to len\(""\)`
	[]int{} = nil // ERROR "cannot assign to \[\]int\{\}"

	var x int = 7

import java.io.File


/**
 * Assign value: T to a property with assign operator
 *
 * @since 8.2
 */
fun <T> Property<T>.assign(value: T?) {
    this.set(value)
}


/**
 * Assign value: Provider<T> to a property with assign operator
 *
 * @since 8.2
 */
fun <T> Property<T>.assign(value: Provider<out T?>) {
    this.set(value)
}


/**

  // returned.
  auto new_result_types = llvm::to_vector<4>(replicate_op->getResultTypes());
  for (auto assign : tail_assign_variable_ops) {
    return_op->insertOperands(return_op->getNumOperands(), assign.getValue());
    new_result_types.insert(new_result_types.end(), num_replicas,
                            assign.getValue().getType());
  }

  OpBuilder builder(replicate_op);

// license that can be found in the LICENSE file.

// Package assign defines an Analyzer that detects useless assignments.
//
// # Analyzer assign
//
// assign: check for useless assignments
//
// This checker reports assignments of the form x = x or a[i] = a[i].
// These are almost always useless, and even when they aren't they are
// usually a mistake.

          scope.WithOpName("var/Read/ReadVariableOp"), var, DataType::DT_FLOAT);
      auto assign = ops::AssignVariableOp(scope.WithOpName("assign"), var, a);
    } else {
      Output var =
          ops::Variable(scope.WithOpName("var"), {}, DataType::DT_FLOAT);
      Output assign = ops::Assign(scope.WithOpName("assign"), var, a);
    }

    TF_ASSERT_OK(scope.ToGraphDef(&graph_def));

			}
			_, b = mib[i]
			if !b {
				panic(fmt.Sprintf("tuple existence assign: mib[%d]\n", i))
			}
		}
		{
			_, b := mii[i]
			if !b {
				panic(fmt.Sprintf("tuple existence decl: mii[%d]\n", i))
			}
			_, b = mii[i]
			if !b {
				panic(fmt.Sprintf("tuple existence assign: mii[%d]\n", i))
			}
		}
		{
			_, b := mfi[f]
			if !b {