[llvm] [LLVM][NVPTX]: Add intrinsic for setmaxnreg (PR #77289)

[Guray Ozen via llvm-commits]

https://github.com/grypp commented:

Looks good to me in general.

PTX doc has restriction for this instruction. Is it possible to check that?

```
Operand imm-reg-count is an integer constant. The value of imm-reg-count must be in the range 24 to 256 (both inclusive) and must be a multiple of 8.
```

https://github.com/llvm/llvm-project/pull/77289