Factor out common ways to call encode from a dfg or func.

lib/cretonne/src/ir/dfg.rs

@@ -1,7 +1,7 @@
 //! Data flow graph tracking Instructions, Values, and EBBs.
 
 use entity::{PrimaryMap, EntityMap};
-use isa::TargetIsa;
+use isa::{TargetIsa, Encoding, Legalize};
 use ir;
 use ir::builder::ReplaceBuilder;
 use ir::extfunc::ExtFuncData;
@@ -645,6 +645,12 @@ impl DataFlowGraph {
             self.value_type(self.first_result(inst))
         }
     }
+
+    /// Wrapper around `TargetIsa::encode` for encoding an existing instruction
+    /// in the `DataFlowGraph`.
+    pub fn encode(&self, inst: Inst, isa: &TargetIsa) -> Result<Encoding, Legalize> {
+        isa.encode(&self, &self[inst], self.ctrl_typevar(inst))
+    }
 }
 
 /// Allow immutable access to instructions via indexing.