[backend-fma]添加浮点乘加融合优化，提供开关简单控制是否启用

src/include/backend/RISCv64/RISCv64Info.h

@@ -85,6 +85,7 @@ static const std::map<RVOpcodes, std::pair<std::vector<int>, std::vector<int>>>
     // --- 浮点指令 ---
     {RVOpcodes::FADD_S, {{0}, {1, 2}}}, {RVOpcodes::FSUB_S, {{0}, {1, 2}}},
     {RVOpcodes::FMUL_S, {{0}, {1, 2}}}, {RVOpcodes::FDIV_S, {{0}, {1, 2}}},
+    {RVOpcodes::FMADD_S, {{0}, {1, 2, 3}}},
     {RVOpcodes::FEQ_S, {{0}, {1, 2}}}, {RVOpcodes::FLT_S, {{0}, {1, 2}}}, {RVOpcodes::FLE_S, {{0}, {1, 2}}},
     {RVOpcodes::FCVT_S_W, {{0}, {1}}}, {RVOpcodes::FCVT_W_S, {{0}, {1}}},
     {RVOpcodes::FCVT_W_S_RTZ, {{0}, {1}}},