Auto merge of #118228 - Mark-Simulacrum:alloc-opt, r=scottmcm

Indicate that multiplication in Layout::array cannot overflow

Since https://github.com/rust-lang/rust/pull/113113, we have added a check that skips calling into the allocator at all if `capacity == 0`. The global, default allocator will not actually try to allocate though; it returns a dangling pointer explicitly. However, these two checks are not merged/deduplicated by LLVM and so we're comparing to zero twice whenever vectors are allocated/grown. Probably cheap, but also potentially expensive in code size and seems like an unfortunate miss.

This removes that extra check by telling LLVM that the multiplication as part of Layout::array can't overflow, turning the original non-zero value into a zero value afterwards. In my checks locally this successfully drops the duplicate comparisons.

See https://rust.godbolt.org/z/b6nPP9dcK for a code example.

```rust
pub fn foo(elements: usize) -> Vec<u32> {
    Vec::with_capacity(elements)
}
```

r? `@scottmcm` since you touched this in a32305a80fd1409b054e97836321bd0621b142fd - curious if you have thoughts on doing this / can confirm my model of this being correct.

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