Move the 'ins' method to DataFlowGraph.

This given us better symmetry between the replace and insert builder operations:

    dfg.replace(inst).iadd(x, y)
    dfg.ins(cursor).imul(x, y)

lib/cretonne/src/ir/dfg.rs

@@ -5,7 +5,8 @@ use ir::entities::{NO_VALUE, ExpandedValue};
 use ir::instructions::{InstructionData, CallInfo};
 use ir::extfunc::ExtFuncData;
 use entity_map::{EntityMap, PrimaryEntityData};
-use ir::builder::ReplaceBuilder;
+use ir::builder::{InsertBuilder, ReplaceBuilder};
+use ir::layout::Cursor;
 
 use std::mem;
 use std::ops::{Index, IndexMut};
@@ -271,6 +272,13 @@ impl DataFlowGraph {
         total_results
     }
 
+    /// Create an `InsertBuilder` that will insert an instruction at the cursor's current position.
+    pub fn ins<'c, 'fc: 'c, 'fd>(&'fd mut self,
+                                 at: &'c mut Cursor<'fc>)
+                                 -> InsertBuilder<'c, 'fc, 'fd> {
+        InsertBuilder::new(self, at)
+    }
+
     /// Create a `ReplaceBuilder` that will replace `inst` with a new instruction in place.
     pub fn replace(&mut self, inst: Inst) -> ReplaceBuilder {
         ReplaceBuilder::new(self, inst)