Auto merge of #113344 - scottmcm:alt-slice-zst-handing, r=the8472

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.

Author: bors