[llvm] [SCEV] Introduce loop-uniform SCEV classification. (PR #194304)

[Martin Storsjö via llvm-commits]

mstorsjo wrote:

I'm also hitting the same issue as above, when compiling libaom and libarchive. Reduced reproducers:
```c
enum { a } * b;
void c() {
  int d, e, f;
  for (;; ++d) {
    e = 0;
    for (;; ++e)
      if (b[e]) {
        f = d;
        for (; f > e; f--)
          b[f] = 1;
        break;
      }
  }
}
```
And
```c
int b, c;
void g() {
  struct {
    int ccc
  } d[10];
  int e, a = b;
  while (a) {
    int f = 0;
    for (; f < e; f++)
      if (d[0].ccc > a)
        break;
    if (f < e) {
      c = e;
      for (; c > f; c--)
        d[c] = d[c - 1];
    }
    e++;
  }
}
```
Both trigger the same way:
```console
$ clang -target x86_64-linux-gnu -c repro.c -O2
Instruction does not dominate all uses!
  %indvar = phi i64 [ %indvar.next, %for.cond ], [ 0, %for.cond.preheader ]
  %smin = call i64 @llvm.smin.i64(i64 %1, i64 %indvar)
clang: ../lib/Transforms/Vectorize/LoopVectorize.cpp:8573: bool llvm::LoopVectorizePass::processLoop(llvm::Loop*): Assertion `!verifyFunction(*F, &dbgs())' failed.
```

I will revert this change soon, as the issue has been reported already 14h ago without any sign on progress.

https://github.com/llvm/llvm-project/pull/194304