Up until now "-0.0 - x" was lowered in the instruction selector. I moved
the lowering now to the MachineOperatorReducer.

I did not remove the lowering from the instruction selector yet, I would
prefer to do that in a separate CL.

R=bmeurer@chromium.org

Review-Url: https://codereview.chromium.org/2226663002
Cr-Commit-Position: refs/heads/master@{#38417}