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.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@47591 91177308-0d34-0410-b5e6-96231b3b80d8
