Incorrect transformation of (sub,neg) to (sub) for fp

A pair (sub,neg) should not be transformed to (sub) for
floating point operations, otherwise we can lose the sign of
zero for instructions like this:
  - (A - B) != B - A if B == A

Change-Id: I4d612612d4dc0a067fac5721ad206f74168bcd36
Signed-off-by: Serguei Katkov <serguei.i.katkov@intel.com>