Consider expression "0.0 - X" as the negation of X if
  - this expression is explicitly marked no-signed-zero, or
  - no-signed-zero of this expression can be derived from some context.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@171922 91177308-0d34-0410-b5e6-96231b3b80d8