[llvm] [SLP]Fix graph traversal in getSpillCost (PR #124984)

[via llvm-commits]

bgra8 wrote:

@alexey-bataev we have too-caused a few `clang` crashes to this revision.

`clang` built at this revision crashes with the `repro.c` and does not crash at the previous version:

```c
char *a, *b;
long h, c;
void *memmove();
void d() {
  long e = h;
  char *f = a, *d = b;
  long double g[1];
  while (e--) {
    memmove(g, f, sizeof(g));
    c = g[0];
    memmove(d, &c, sizeof(c));
    d += sizeof(long);
  }
}
```

Compilation command line:

```
$ clang -O2 \
  --target=aarch64-generic-linux-gnu \
  -mcpu=neoverse-n1 \
  -mtune=generic \
  -fno-strict-float-cast-overflow \
  -c repro.c \
  -o /tmp/repro.o
```

Can you please revert to green and fix?


https://github.com/llvm/llvm-project/pull/124984