[LoongArch] Support llvm.is.fpclass for f32 and f64

is_fpclass (fj, mask)
->
sltu (r0, and (movfr2gr.[sd] (fclass.[sd] fj), (to_fclass_mask mask)))

[1]: https://loongson.github.io/LoongArch-Documentation/LoongArch-Vol1-EN.html#_fclass_sd

Reviewed By: wangleiat

Differential Revision: https://reviews.llvm.org/D159183