[LoongArch] Fix implicit PesudoXVINSGR2VR error (#152432)
According to the instructions manual, when `vr0` is changed, high 128
bit of `xr0` is undefined.
Use `vinsgr2vr.b/h` to insert an `i8/i16` to low 128bit of a 256 vector
may cause undefined behavior when high 128bit is used in later
instructions.
