Add source location support to FuncCursor and EncCursor.

A cursor now also remembers a current source location which will be
assigned to all new instructions created with the cursor.

The old layout::Cursor can't support source locations because it doesn't
have a reference to the full ir::Function.

lib/cretonne/src/regalloc/coloring.rs

@@ -148,8 +148,9 @@ impl<'a> Context<'a> {
         self.divert.clear();
 
         // Now go through the instructions in `ebb` and color the values they define.
-        let mut pos = Cursor::new(&mut func.layout).at_top(ebb);
+        let mut pos = Cursor::new(&mut func.layout, &mut func.srclocs).at_top(ebb);
         while let Some(inst) = pos.next_inst() {
+            pos.use_srcloc(inst);
             if let Some(constraints) = self.encinfo.operand_constraints(func.encodings[inst]) {
                 self.visit_inst(
                     inst,