[Frontend] Interface fixes for libdevice (#830)

- Unifying several interfaces with different types to a single one, e.g.
`fsub_ru` and `dsub_ru` -> `sub_ru`;
- Minor bug fix: `fast_pow` is incorrectly classified into the `pow`
interface, of which arguments are the same as `powf`;
- Explicit interfaces for casting functions, e.g. decoupling
`ll2float_ru` to `ll2float_ru` and `ull2float_ru`;
- Removing interfaces that are not in NVIDIA's official documents, e.g.
`fmaf_ieee_rn`, which is confusing together with `fmaf_rn`.

Note that this PR for the master branch is different from #829, which is
for the MLIR branch.

python/tutorials/07-libdevice-function.py

@@ -7,7 +7,7 @@ Please refer to https://docs.nvidia.com/cuda/libdevice-users-guide/index.html re
 
 In `trition/language/libdevice.py`, we try to aggregate functions with the same computation but different data types together.
 For example, both `__nv_asin` and `__nvasinf` calculate the principal value of the arc sine of the input, but `__nv_asin` operates on `double` and `__nv_asinf` operates on `float`.
-Using triton, you can simply call `tl.libdevice.asinf`.
+Using triton, you can simply call `tl.libdevice.asin`.
 triton automatically selects the correct underlying device function to invoke based on input and output types.
 """