[llvm] [ValueTracking] Propagate sign information out of loop (PR #175590)

[Martin Storsjö via llvm-commits]

mstorsjo wrote:

I’m also hitting issues here. Reduced C code:
```c
double b;
void a();
long double modfl() {
  long double c;
  if (modfl(0, &c))
    modfl(b, &c);
  if (c)
    a();
}
```
```console
$ clang -target x86_64-linux-gnu -c -O2 repro.c
```

https://github.com/llvm/llvm-project/pull/175590