Fix/ref prop preserve field alignment

[YunusMutlu]

Fixes #164599

Summary

Problem: A read through a raw u32 field pointer can be transformed by ReferencePropagation into an access through a misaligned Aligned8 pointer.

Fix: This change adds a conservative guard that prevents potentially unsafe raw-pointer field projections from being recorded as replacement targets.

Validation: I reproduced the align 8 versus align 4 difference on both stable and nightly Rust. I also ran the codegen regression tests, the negative MIR test, and the positive reborrow test.

Limitation: The guard may prevent some otherwise safe optimizations. Its performance impact has not yet been measured.

[rustbot]

Some changes occurred to MIR optimizations

cc @rust-lang/wg-mir-opt

[rustbot]

Thanks for the pull request, and welcome! The Rust Project has assigned @jieyouxu (or someone else) to review your changes, you should hear from them (or someone else) within the next two weeks.

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[jieyouxu]

r? mir-opt