Add a func_addr instruction.

Get the callable address of a function. Use for long distance calls and
for creating arguments to call_indirect in general.

lib/cretonne/src/ir/instructions.rs

@@ -178,6 +178,7 @@ pub enum InstructionData {
         sig_ref: SigRef,
         args: ValueList,
     },
+    FuncAddr { opcode: Opcode, func_ref: FuncRef },
     StackLoad {
         opcode: Opcode,
         stack_slot: StackSlot,

lib/cretonne/src/verifier/mod.rs

@@ -306,6 +306,9 @@ impl<'a> Verifier<'a> {
                 self.verify_sig_ref(inst, sig_ref)?;
                 self.verify_value_list(inst, args)?;
             }
+            FuncAddr { func_ref, .. } => {
+                self.verify_func_ref(inst, func_ref)?;
+            }
             StackLoad { stack_slot, .. } |
             StackStore { stack_slot, .. } => {
                 self.verify_stack_slot(inst, stack_slot)?;

lib/cretonne/src/write.rs

@@ -346,6 +346,7 @@ pub fn write_operands(
                 DisplayValues(&args[1..])
             )
         }
+        FuncAddr { func_ref, .. } => write!(w, " {}", func_ref),
         StackLoad { stack_slot, offset, .. } => write!(w, " {}{}", stack_slot, offset),
         StackStore {
             arg,