Rollup merge of #111282 - scottmcm:remove-unneeded-assumes, r=workingjubilee

Remove some `assume`s from slice iterators that don't do anything

Because the start pointer is iterators is already a `NonNull`, we emit the appropriate `!nonnull` metadata when loading the pointer to tell LLVM that it's non-null.

Probably the best way to see that it's the metadata that's important (and not the `assume`) is to observe that LLVM actually *removes* the `assume` from the optimized IR: <https://rust.godbolt.org/z/KhE6G963n>.

(I also checked that, yes, the if-not-ZST `assume` on `end` is still doing something: it's how there's a `!nonnull` metadata on its load, even though it's an ordinary raw pointer.  The codegen test added in this PR fails if the other `assume` is  removed.)

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