Auto merge of #116510 - scottmcm:no-1-simd-v2, r=compiler-errors

Copy 1-element arrays as scalars, not vectors

For `[T; 1]` it's silly to copy as `<1 x T>` when we can just copy as `T`.

Inspired by rust-lang/rust#101210 (comment), which pointed out that `Option<[u8; 1]>` was codegenning worse than `Option<u8>`.

(I'm not sure *why* LLVM doesn't optimize out `<1 x u8>`, but might as well just not emit it in the first place in this codepath.)

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I think I bit off too much in #116479; let me try just the scalar case first.

r? `@ghost`