This CL changes the semantics of FloatXXSub to match the semantics of
the semantics of FloatXXSubPreserveNan. Therefore there is no need
anymore for the FloatXXSubPreserveNan operators.

The optimizations in VisitFloatXXSub which are removed in this CL have
already been moved to machine-operator-reducer.cc in
https://codereview.chromium.org/2226663002

R=bmeurer@chromium.org

Review-Url: https://codereview.chromium.org/2220973002
Cr-Commit-Position: refs/heads/master@{#38437}