Get `!nonnull` metadata on slice iterators, without `assume`s
This updates the non-ZST paths to read the end pointer through a pointer-to-`NonNull`, so that they all get `!nonnull` metadata.
That means that the last `assume(!ptr.is_null())` can be deleted, without impacting codegen -- the codegen tests confirm the LLVM-IR ends up exactly the same as before.
It makes it sound like the `ExprKind` and `Rvalue` are supposed to represent all pointer related
casts, when in reality their just used to share a some enum variants. Make it clear there these
are only coercion to make it clear why only some pointer related "casts" are in the enum.
There's no need for the `const_eval_select` dance here. And while I originally wrote the `.try_into().unwrap_unchecked()` implementation here, it's kinda a mess in MIR -- this new one is substantially simpler, as shown by the old one being above the inlining threshold but the new one being below it.
To reproduce the changes in this commit locally:
- Run `./x test tidy` and remove all the output files not associated
with a test file anymore, as reported by tidy.
- Run `./x test tests/mir-opt --bless` to generate the new outputs.
All the implementations of the trait already are `Copy`, and this seems to be enough to simplify the implementations enough to make the MIR inliner willing to inline basics like `Range::next`.
Since this only affects `PreCodegen MIR, and it would be nice for that to be resilient to permutations of things that don't affect the actual semantic behaviours.
