Rollup merge of #145585 - RalfJung:miri-inplace-arg-checks, r=compiler-errors

Miri: fix handling of in-place argument and return place handling

This fixes two separate bugs (in two separate commits):
- If the return place is `_local` and not `*ptr`, we didn't always properly protect it if there were other pointers pointing to that return place.
- If two in-place arguments are *the same* local variable, we didn't always detect that aliasing.