feat(binder): add const case-when evaluation optimization during binding

[xzhseh]

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What's changed and what's your intention?

As titled, the optimization goes to case-when expression in below format, e.g.,

select case <constant> 
    when <constant> then <expr_1>
    when <constant> then <expr_2>
    ... (potentially lots of other arms with constant condition clause)
    else <expr_fallback> end;

related: #14586 (comment).

And since I'm new to optimizer, feel free to point out any part I may have missed and any suggestion is welcomed. 😄

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[xzhseh]

The reason I create another check function is that we can't check all the optimizable case-when patterns within only one pass, and they indeed have subtle but significant difference, which may not be generalized.

[chenzl25]

Can we just return constant_case_when_eval_inputs[0] here? Then we don't even need to use a rule to convert it later.

[xzhseh]

Then what type should we return, I doubt FunctionCall would work 🤔, also Literal may not be the appropriate one.

[chenzl25]

This method returns an ExprImpl, an enum that could be any expression type.

[xzhseh]

Got it!
Then I think return a bound expression should work, and I'll remove the relevant rules. 😄

[chenzl25]

LGTM