Add a BinaryOverflow instruction format.

This will eventualy be used for add-with-carry and add-with-overflow type
instructions. For now it only serves as a representative of instruction formats
that have multiple_results=True and boxed_storage=False at the same time.

cranelift/src/libcretonne/instructions.rs

@@ -133,13 +133,19 @@ pub enum InstructionData {
         lhs: Value,
         rhs: Imm64,
     },
-    // Same as BinaryImm, but the imediate is the lhs operand.
+    // Same as BinaryImm, but the immediate is the lhs operand.
     BinaryImmRev {
         opcode: Opcode,
         ty: Type,
         rhs: Value,
         lhs: Imm64,
     },
+    BinaryOverflow {
+        opcode: Opcode,
+        ty: Type,
+        second_result: Value,
+        args: [Value; 2],
+    },
     Jump {
         opcode: Opcode,
         ty: Type,