RFC: Generate select instruction for conditional returns.

The select generator currently only inserts select instructions
if there is a diamond shape with a phi.

This change extends the select generator to also deal with the
pattern:

  if (condition) {
    movable instruction 0
    return value0
  } else {
    movable instruction 1
    return value1
  }

which it turns into:

  moveable instruction 0
  moveable instruction 1
  return select (value0, value1, condition)

Test: 592-checker-regression-bool-input
Change-Id: Iac50fb181dc2c9b7619f28977298662bc09fc0e1