Remove (lots of) dead code

Found with https://github.com/est31/warnalyzer.

Dubious changes:
- Is anyone else using rustc_apfloat? I feel weird completely deleting
  x87 support.
- Maybe some of the dead code in rustc_data_structures, in case someone
  wants to use it in the future?
- Don't change rustc_serialize

  I plan to scrap most of the json module in the near future (see
  https://github.com/rust-lang/compiler-team/issues/418) and fixing the
  tests needed more work than I expected.

TODO: check if any of the comments on the deleted code should be kept.

compiler/rustc_query_system/src/dep_graph/debug.rs

@@ -52,6 +52,7 @@ impl EdgeFilter {
         }
     }
 
+    #[cfg(debug_assertions)]
     pub fn test<K: DepKind>(&self, source: &DepNode<K>, target: &DepNode<K>) -> bool {
         self.source.test(source) && self.target.test(target)
     }

compiler/rustc_query_system/src/dep_graph/graph.rs

@@ -488,8 +488,8 @@ impl<K: DepKind> DepGraph<K> {
         self.data.is_some() && self.dep_node_index_of_opt(dep_node).is_some()
     }
 
-    #[inline]
-    pub fn dep_node_of(&self, dep_node_index: DepNodeIndex) -> DepNode<K> {
+    #[cfg(debug_assertions)]
+    fn dep_node_of(&self, dep_node_index: DepNodeIndex) -> DepNode<K> {
         let data = self.data.as_ref().unwrap();
         let previous = &data.previous;
         let data = data.current.data.lock();

compiler/rustc_query_system/src/dep_graph/prev.rs

@@ -35,11 +35,6 @@ impl<K: DepKind> PreviousDepGraph<K> {
         self.data.nodes[dep_node_index]
     }
 
-    #[inline]
-    pub fn node_to_index(&self, dep_node: &DepNode<K>) -> SerializedDepNodeIndex {
-        self.index[dep_node]
-    }
-
     #[inline]
     pub fn node_to_index_opt(&self, dep_node: &DepNode<K>) -> Option<SerializedDepNodeIndex> {
         self.index.get(dep_node).cloned()

compiler/rustc_query_system/src/dep_graph/query.rs

@@ -31,10 +31,6 @@ impl<K: DepKind> DepGraphQuery<K> {
         DepGraphQuery { graph, indices }
     }
 
-    pub fn contains_node(&self, node: &DepNode<K>) -> bool {
-        self.indices.contains_key(&node)
-    }
-
     pub fn nodes(&self) -> Vec<&DepNode<K>> {
         self.graph.all_nodes().iter().map(|n| &n.data).collect()
     }