[SLP] Make getSameOpcode support interchangeable instructions. (#133888)

We use the term "interchangeable instructions" to refer to different
operators that have the same meaning (e.g., `add x, 0` is equivalent to
`mul x, 1`).
Non-constant values are not supported, as they may incur high costs with
little benefit.

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Co-authored-by: Alexey Bataev <a.bataev@gmx.com>