[libclc] [libclc] Replace NVPTX libdevice calls with clang builtins for sqrt, rsqrt, isinf (PR #205709)

[Joseph Huber via cfe-commits]

jhuber6 wrote:

Well, I suppose technically there's the weird FTZ handling. I don't know if libclc wants to copy this, the nvvm_reflect calls basically take compile flags and statically lower it before reaching the backend, really hacky.
```llvm
define float @__nv_rsqrtf(float %x) #0 {
  %1 = call i32 @__nvvm_reflect(ptr @.str)
  %2 = icmp ne i32 %1, 0
  br i1 %2, label %3, label %5

3:                                                ; preds = %0
  %4 = call float @llvm.nvvm.rsqrt.approx.ftz.f(float %x)
  br label %7

5:                                                ; preds = %0
  %6 = call float @llvm.nvvm.rsqrt.approx.f(float %x)
  br label %7

7:                                                ; preds = %5, %3
  %.0 = phi float [ %4, %3 ], [ %6, %5 ]
  ret float %.0
}
```

https://github.com/llvm/llvm-project/pull/205709