Fix constant fold of div by zero and rem by zero to match IEEE 754

Fix constant fold of div by zero and rem by zero to match IEEE 754
requirements. We must return NaN in some cases and correctly signed
infinity in other cases. Passes CFP2006 (not that that says much).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@35277 91177308-0d34-0410-b5e6-96231b3b80d8