fix(frontend): 规范一下前端实现

src/irgen/IRGenDecl.cpp

@@ -6,15 +6,34 @@
 #include "ir/IR.h"
 
 void IRGenImpl::GenBlock(SysYParser::BlockContext& block) {
-  for (auto* stmt : block.stmt()) {
-    if (stmt) {
-      if (GenStmt(*stmt)) {
+  for (auto* item : block.blockItem()) {
+    if (item) {
+      if (GenBlockItem(*item)) {
         break;
       }
     }
   }
 }
 
+bool IRGenImpl::GenBlockItem(SysYParser::BlockItemContext& item) {
+  if (item.decl()) {
+    GenDecl(*item.decl());
+    return false;
+  }
+  if (item.stmt()) {
+    return GenStmt(*item.stmt());
+  }
+  throw std::runtime_error("[irgen] 暂不支持的 blockItem 类型");
+}
+
+void IRGenImpl::GenDecl(SysYParser::DeclContext& decl) {
+  if (decl.varDecl()) {
+    GenVarDecl(*decl.varDecl());
+    return;
+  }
+  throw std::runtime_error("[irgen] 暂不支持的声明类型");
+}
+
 void IRGenImpl::GenVarDecl(SysYParser::VarDeclContext& decl) {
   const std::string name = decl.Ident()->getText();
   if (locals_.find(name) != locals_.end()) {