Rollup merge of #111525 - scottmcm:slice-position-tweak, r=Mark-Simulacrum

Stop checking for the absence of something that doesn't exist

A couple of codegen tests are doing
```
// CHECK-NOT: slice_index_len_fail
```

However, that function no longer exists: [the only places](https://github.com/search?q=repo%3Arust-lang%2Frust+slice_index_len_fail&type=code) it occurs in the repo are in those tests.

So this PR updates the tests to check for the absense of the functions that are actually used today to panic for out-of-bounds indexing.

Author: matthiaskrgr