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/legalizer/split.rs

@@ -120,7 +120,7 @@ fn split_any(
     concat: Opcode,
 ) -> (Value, Value) {
     let mut repairs = Vec::new();
-    let mut pos = ir::Cursor::new(layout).at_position(pos);
+    let mut pos = ir::Cursor::new(layout, None).at_position(pos);
     let result = split_value(dfg, &mut pos, value, concat, &mut repairs);
 
     // We have split the value requested, and now we may need to fix some EBB predecessors.