Auto merge of #108623 - scottmcm:try-different-as-slice-impl, r=the8472

Move `Option::as_slice` to an always-sound implementation

This approach depends on CSE to not have any branches or selects when the guessed offset is correct -- which it always will be right now -- but to also be *sound* (just less efficient) if the layout algorithms change such that the guess is incorrect.

The codegen test confirms that CSE handles this as expected, leaving the optimal codegen.

cc JakobDegen #108545

Author: bors