[backend] fix bugs of not

src/SysYIRGenerator.cpp

@@ -343,6 +343,7 @@ std::any SysYIRGenerator::visitIfStmt(SysYParser::IfStmtContext *ctx) {
       ctx->stmt(1)->accept(this);
       module->leaveScope();
     }
+    builder.createUncondBrInst(exitBlock, {});
     BasicBlock::conectBlocks(builder.getBasicBlock(), exitBlock);
 
     labelstring << "exit.L" << builder.getLabelIndex();