Reassociate x + -0.1234 * y into x - 0.1234 * y

This does not require -ffast-math, and it gives CSE/GVN more options to
eliminate duplicate expressions in, e.g.:

  return ((x + 0.1234 * y) * (x - 0.1234 * y));

Differential Revision: http://reviews.llvm.org/D4904