Make the dfg.insts table private again.

- Add a dfg.is_inst_valid() method for the verifier.
- Use the inst_args_mut() method when rewriting values in the parser.
- Add a new branch_destination_mut() to use when rewriting EBBs.

This also gets rid of one of the large instruction format switches in
the parser.

lib/cretonne/src/ir/dfg.rs

@@ -26,7 +26,7 @@ pub struct DataFlowGraph {
     /// Data about all of the instructions in the function, including opcodes and operands.
     /// The instructions in this map are not in program order. That is tracked by `Layout`, along
     /// with the EBB containing each instruction.
-    pub insts: EntityMap<Inst, InstructionData>,
+    insts: EntityMap<Inst, InstructionData>,
 
     /// Memory pool of value lists referenced by instructions in `insts`.
     pub value_lists: ValueListPool,
@@ -77,6 +77,11 @@ impl DataFlowGraph {
         self.insts.len()
     }
 
+    /// Returns `true` if the given instruction reference is valid.
+    pub fn inst_is_valid(&self, inst: Inst) -> bool {
+        self.insts.is_valid(inst)
+    }
+
     /// Get the total number of extended basic blocks created in this function, whether they are
     /// currently inserted in the layout or not.
     ///