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
diff --git a/compiler/optimizing/nodes.h b/compiler/optimizing/nodes.h
index 488d472..0b47574 100644
--- a/compiler/optimizing/nodes.h
+++ b/compiler/optimizing/nodes.h
@@ -959,6 +959,7 @@
   }
 
   bool IsSingleGoto() const;
+  bool IsSingleReturn() const;
   bool IsSingleTryBoundary() const;
 
   // Returns true if this block emits nothing but a jump.