SC: operands evaluated multiple times for <=> operator on reference types

Similar to GROOVY-7473, consider the following:

@groovy.transform.CompileStatic
class C {
  int i = 0, j = 1
  Integer getA() { i++ }
  Integer getB() { j++ }
  void test() {
    assert (a <=> b) == -1
    print i
    print j
  }
}
new C().test()

In cases where "a" and "b" are not identical and non-null, each is evaluated 3 times (script prints "34" not "12"). This is not good if "a" and "b" are expensive to compute or have side effects.

BinaryExpressionTransformer converts a <=> b into a === b ? 0 : (a == null ? -1 : (b == null ? 1 : a.compareTo(b)))