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.
     ///

lib/cretonne/src/ir/instructions.rs

@@ -376,6 +376,19 @@ impl InstructionData {
         }
     }
 
+    /// Get a mutable reference to the single destination of this branch instruction, if it is a
+    /// single destination branch or jump.
+    ///
+    /// Multi-destination branches like `br_table` return `None`.
+    pub fn branch_destination_mut(&mut self) -> Option<&mut Ebb> {
+        match *self {
+            InstructionData::Jump { ref mut destination, .. } => Some(destination),
+            InstructionData::Branch { ref mut destination, .. } => Some(destination),
+            InstructionData::BranchIcmp { ref mut destination, .. } => Some(destination),
+            _ => None,
+        }
+    }
+
     /// Return information about a call instruction.
     ///
     /// Any instruction that can call another function reveals its call signature here.

lib/cretonne/src/verifier.rs

@@ -337,7 +337,7 @@ impl<'a> Verifier<'a> {
         match dfg.value_def(v) {
             ValueDef::Res(def_inst, _) => {
                 // Value is defined by an instruction that exists.
-                if !dfg.insts.is_valid(def_inst) {
+                if !dfg.inst_is_valid(def_inst) {
                     return err!(loc_inst,
                                 "{} is defined by invalid instruction {}",
                                 v,