[PATCH] Reassociate x + -0.1234 * y into x - 0.1234 * y

[Chad Rosier]

> Hi Erik,
> My suggestions were easier to see in a patch, so see attached. I cleaned
> it up a bit then ran it through clang-format.  NFCI.  If you'd like to
> add/edit anything, please do.

A few more things..

// This is only done for non-fast-math, as fast-math is handled separately.

My recent patch added support for fast-math in the reassociation pass.  If
this is safe for non-fast-math, then it should be fine for fast-math as
well.  Thus, you might consider removing this check:

 !cast<BinaryOperator>(I)->getFastMathFlags().any()

A few other questions.  Given reassociation now works with fast-math,
1. Are we sure your patch is still necessary?
2. Is your patch strictly improving things?

  Chad