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

[Alexey Bataev via llvm-commits]

alexey-bataev wrote:

> @alexey-bataev we (at google) have root-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?

The crash is not related to SLP vectorizer, the reproducer crashes in codegen. I suggest to create an issue again AARCH64 codegen

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