Fix isNegatibleForFree to not return true for ConstantFP nodes

after legalize.  Just because a constant is legal (e.g. 0.0 in SSE) 
doesn't mean that its negated value is legal (-0.0).  We could make
this stronger by checking to see if the negated constant is actually
legal post negation, but it doesn't seem like a big deal.

llvm-svn: 47591