This change is to implement following rules:

  o. X/C1 * C2 => X * (C2/C1) (if C2/C1 is neither special FP nor denormal)
  o. X/C1 * C2 -> X/(C1/C2)   (if C2/C1 is either specical FP or denormal, but C1/C2 is a normal Fp)

     Let MDC denote multiplication or dividion with one & only one operand being a constant
  o. (MDC ± C1) * C2 => (MDC * C2) ± (C1 * C2)
     (so long as the constant-folding doesn't yield any denormal or special value)

llvm-svn: 171793