partly fix issue 8525

- Only perform associative 'a op1 (b op2 c) => (a op2 b) op1 c'
  transformation if 'b' is not a constant. This breaks a cycle
  with the '(a op c1) op c2 => a op (c1 op c2)' transformation
  if folding 'c1 op c2' fails.

http://d.puremagic.com/issues/show_bug.cgi?id=8525