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.
One extension worth noting is the use of revisions as custom prefixes for
FileCheck. If your codegen test has different behavior based on the chosen
target or different compiler flags that you want to exercise, you can use a
revisions annotation, like so:
After specifying those variations, you can write different expected, or
explicitly unexpected output by using <prefix>-SAME: and <prefix>-NOT:,
like so: