port f8938e50 (r38437)

  original commit message:
  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

BUG=

Review-Url: https://codereview.chromium.org/2236153002
Cr-Commit-Position: refs/heads/master@{#38571}