operations in the while loop body are dependent on which inputs, it captures
inter iteration parallelism in while loop. Control dependencies on the other
hand create a barrier at the end of while loop body thus blocking any
parallelism across iterations.

For example, the following while loop body has a `%barrier` at the end.
Although there is no data/control dependency between `tf.AssignVariableOp`

package ssa

import (
	"fmt"
)

type loop struct {
	header *Block // The header node of this (reducible) loop
	outer  *loop  // loop containing this loop

	// By default, children, exits, and depth are not initialized.
	children []*loop  // loops nested directly within this loop. Initialized by assembleChildren().
	exits    []*Block // exits records blocks reached by exits from this loop. Initialized by findExits().

package ssa

// loopRotate converts loops with a check-loop-condition-at-beginning
// to loops with a check-loop-condition-at-end.
// This helps loops avoid extra unnecessary jumps.
//
//	 loop:
//	   CMPQ ...
//	   JGE exit
//	   ...
//	   JMP loop
//	 exit:
//
//	  JMP entry
//	loop:
//	  ...
//	entry:
//	  CMPQ ...
//	  JLT loop
func loopRotate(f *Func) {
	loopnest := f.loopnest()

  for (int i = 0; i < n; ++i) result.push_back(ToOutput(output_tensors[i]));
  return result;
}

// The backprop loop counter and main backprop loop run in their own execution
// frame (conceptually, the main forward loop and forward loop counter run
// together in a frame, then the backprop loop counter and backprop loop run
// together in a different frame). This returns the frame name to use for the
// backprop while loops.

// CHECK-LABEL: func @preserve_unreachable_tpu_replicate_metadata
func.func @preserve_unreachable_tpu_replicate_metadata() {
  tf_executor.graph {
    %0 = tf_executor.ControlTrigger {}
    // CHECK: "tf.NoOp"
    %1 = tf_executor.island wraps "tf.NoOp"() : () -> ()
    // CHECK: "tf.TPUReplicateMetadata"

        code << [
                "id('noop')",
                "id 'noop'",
                "id('noop').version('bar')",
                "id 'noop' version 'bar'",
                "id('noop').\nversion 'bar'",
                "id('java');id('noop')",
                "id('java')\nid('noop')",
                "id('noop').version('bar');id('java')",
                "id('noop').version('bar')\nid('java')",

// Package loopclosure defines an Analyzer that checks for references to
// enclosing loop variables from within nested functions.
//
// # Analyzer loopclosure
//
// loopclosure: check references to loop variables from within nested functions
//
// This analyzer reports places where a function literal references the
// iteration variable of an enclosing loop, and the loop calls the function
// in such a way (e.g. with go or defer) that it may outlive the loop

package escape

var x bool

func f1() {
	var p *int
loop:
	if x {
		goto loop
	}
	// BAD: We should be able to recognize that there
	// aren't any more "goto loop" after here.
	p = new(int) // ERROR "escapes to heap"
	_ = p
}

func f2() {
	var p *int
	if x {
	loop:
		goto loop
	} else {
		p = new(int) // ERROR "does not escape"
	}
	_ = p
}

func f3() {

func main() {
	var i, k int
	var r string
outer:
	for k = 0; k < 2; k++ {
		r += fmt.Sprintln("outer loop top k", k)
		if k != 0 {
			panic("k not zero")
		} // inner loop breaks this one every time
		for i = 0; i < 2; i++ {
			if i != 0 {
				panic("i not zero")
			} // loop breaks every time
			r += fmt.Sprintln("inner loop top i", i)
			if true {
				r += "do break\n"
				break outer
			}
		}
	}
	r += "broke\n"

    BodyGraphBuilderFn;

// Constructs a while loop.
//
// Arguments:
// * scope: used to construct the while loop.
// * inputs: the initial values of the loop variables. Must be non-empty.
// * cond: a function that builds the condition graph of the loop. Takes the
//     current loop variables as inputs and returns a scalar boolean Output
//     indicating whether the loop should continue.