BinOpAssign always returns unit

[oli-obk]

I don't know why we treated assign ops as returning their binop type sometimes, but it's usually ignored later anyway and mostly affects infer vars.

Also updated a comment from 11 years ago when SIMD types apparently had builtin == logic.

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[nnethercote]

@bors r+ rollup

[rust-bors]

📌 Commit 3be0e76 has been approved by nnethercote

It is now in the queue for this repository.