[LLVM][NVPTX] Remove nonexistent ftz ops (#106100)
According to the PTX
[spec](https://docs.nvidia.com/cuda/parallel-thread-execution/#half-precision-floating-point-instructions-max),
max & min instructions do not support the `ftz` modifier for `bf16` &
`bf16x2` types. This PR removes them from instr info, and the non-ftz
legal versions will be emitted instead.
