Inline `NodeIndex` methods.

Because they are small and hot.

src/librustc_data_structures/obligation_forest/node_index.rs

@@ -17,11 +17,13 @@ pub struct NodeIndex {
 }
 
 impl NodeIndex {
+    #[inline]
     pub fn new(value: usize) -> NodeIndex {
         assert!(value < (u32::MAX as usize));
         NodeIndex { index: NonZeroU32::new((value as u32) + 1).unwrap() }
     }
 
+    #[inline]
     pub fn get(self) -> usize {
         (self.index.get() - 1) as usize
     }