Merge return place with other locals in CopyProp.

This reintroduces a limited form of NRVO.

r? wg-mir-opt

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.

allow mutating function args through `&raw const`

Fixes https://github.com/rust-lang/rust/issues/111502 by "turning off the sketchy optimization while we figure out if this is ok", like `@JakobDegen` said.

The first commit in this PR removes some suspicious looking logic from the same method, but should have no functional changes, since it doesn't modify the `context` outside of the method. Best reviewed commit by commit.

r? opsem

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.

CFI: Fix SIGILL reached via trait objects

Fix #106547 by transforming the concrete self into a reference to a trait object before emitting type metadata identifiers for trait methods.

vec-shrink-panik: update expectations to work on LLVM 17

For some reason, the called function is `cleanup` on LLVM 17 instead of `filter`.

r? `@Amanieu`

For some reason, the called function is `cleanup` on LLVM 17 instead of
`filter`.

r? @Amanieu