Port f8938e50

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

R=ahaas@chromium.org, joransiu@ca.ibm.com, jyan@ca.ibm.com, michael_dawson@ca.ibm.com, mbrandy@us.ibm.com

BUG=
LOG=N

Review-Url: https://codereview.chromium.org/2228973002
Cr-Commit-Position: refs/heads/master@{#38524}