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/layout.rs

@@ -6,8 +6,6 @@
 use std::iter::{Iterator, IntoIterator};
 use entity_map::{EntityMap, EntityRef};
 use ir::entities::{Ebb, NO_EBB, Inst, NO_INST};
-use ir::dfg::DataFlowGraph;
-use ir::builder::InsertBuilder;
 
 /// The `Layout` struct determines the layout of EBBs and instructions in a function. It does not
 /// contain definitions of instructions or EBBs, but depends on `Inst` and `Ebb` entity references
@@ -617,11 +615,6 @@ impl<'f> Cursor<'f> {
         }
     }
 
-    /// Create a builder for inserting an instruction at the current position.
-    pub fn ins<'c, 'fd>(&'c mut self, dfg: &'fd mut DataFlowGraph) -> InsertBuilder<'c, 'f, 'fd> {
-        InsertBuilder::new(dfg, self)
-    }
-
     /// Insert an EBB at the current position and switch to it.
     ///
     /// As far as possible, this method behaves as if the EBB header were an instruction inserted