Generate LLVM undef on CPU

• Lower Undef node to undef in LLVM for CPU backend.
• TODO: decide whether it's more appropriate to use undef or poison?

hercules_cg/src/cpu.rs

@@ -536,6 +536,17 @@ impl<'a> CPUContext<'a> {
                     self.get_value(collect, true)
                 )?;
             }
+            Node::Undef { ty } => {
+                let body = &mut blocks.get_mut(&self.bbs.0[id.idx()]).unwrap().body;
+                let ty = self.get_type(ty);
+                write!(
+                    body,
+                    "  {} = bitcast {} undef to {}\n",
+                    self.get_value(id, false),
+                    ty,
+                    ty
+                )?;
+            }
             _ => panic!("PANIC: Can't lower {:?}.", self.function.nodes[id.idx()]),
         }
         Ok(())