Avoid various uses of Option<Span> in favor of using DUMMY_SP in the few cases that used None

compiler/rustc_mir_transform/src/dataflow_const_prop.rs

@@ -394,7 +394,7 @@ impl<'a, 'tcx> ConstAnalysis<'a, 'tcx> {
             }
             Operand::Constant(box constant) => {
                 if let Ok(constant) =
-                    self.ecx.eval_mir_constant(&constant.const_, Some(constant.span), None)
+                    self.ecx.eval_mir_constant(&constant.const_, constant.span, None)
                 {
                     self.assign_constant(state, place, constant, &[]);
                 }