Use PackedOption<Value> instead of NO_VALUE.

- Remove NO_VALUE and ExpandedValue::None.
- Remove the Default implelmentation for Value.
- InstructionData::second_result() returns an Option<Value>.
- InstructionData::second_result() returns a reference to the packed
  option.

lib/cretonne/src/ir/instructions.rs

@@ -16,6 +16,7 @@ use ir::condcodes::*;
 use ir::types;
 
 use ref_slice::*;
+use packed_option::PackedOption;
 
 // Include code generated by `lib/cretonne/meta/gen_instr.py`. This file contains:
 //
@@ -126,7 +127,7 @@ pub enum InstructionData {
     UnarySplit {
         opcode: Opcode,
         ty: Type,
-        second_result: Value,
+        second_result: PackedOption<Value>,
         arg: Value,
     },
     Binary {
@@ -150,7 +151,7 @@ pub enum InstructionData {
     BinaryOverflow {
         opcode: Opcode,
         ty: Type,
-        second_result: Value,
+        second_result: PackedOption<Value>,
         args: [Value; 2],
     },
     Ternary {
@@ -161,7 +162,7 @@ pub enum InstructionData {
     TernaryOverflow {
         opcode: Opcode,
         ty: Type,
-        second_result: Value,
+        second_result: PackedOption<Value>,
         data: Box<TernaryOverflowData>,
     },
     InsertLane {
@@ -207,13 +208,13 @@ pub enum InstructionData {
     Call {
         opcode: Opcode,
         ty: Type,
-        second_result: Value,
+        second_result: PackedOption<Value>,
         data: Box<CallData>,
     },
     IndirectCall {
         opcode: Opcode,
         ty: Type,
-        second_result: Value,
+        second_result: PackedOption<Value>,
         data: Box<IndirectCallData>,
     },
     Return {