Rollup merge of #133700 - RalfJung:const-non-null, r=lcnr

const-eval: detect more pointers as definitely not-null

This fixes https://github.com/rust-lang/rust/issues/133523 by making the `scalar_may_be_null` check smarter: for instance, an odd offset in any 2-aligned allocation can never be null, even if it is out-of-bounds.

More generally, if an allocation with unknown base address B is aligned to alignment N, and a pointer is at offset X inside that allocation, then we know that `(B + X) mod N = B mod N + X mod N = X mod N`. Since `0 mod N` is definitely 0, if we learn that `X mod N` is *not* 0 we can deduce that `B + X` is not 0.

This is immediately visible on stable, via `ptr.is_null()` (and, more subtly, by not raising a UB error when such a pointer is used somewhere that a non-null pointer is required). Therefore nominating for `@rust-lang/lang.`

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