regexp/syntax: optimize OpCharClass fold case in calcFlags

[func25]

Fixes #69303

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Message from Gopher Robot:

Patch Set 1:

(1 comment)

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Message from Ian Lance Taylor:

Patch Set 2: Commit-Queue+1

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Message from Go LUCI:

Patch Set 2:

Dry run: CV is trying the patch.

Bot data: {"action":"start","triggered_at":"2024-09-06T04:51:55Z","revision":"45dce056f9c76f4cce923b491a39ead55454963b"}

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Message from Ian Lance Taylor:

Patch Set 2: -Commit-Queue

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Patch Set 2:

This CL has passed the run

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Patch Set 2: LUCI-TryBot-Result+1

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Message from Phuong Le (Aiden):

Patch Set 3:

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Message from Cuong Manh Le:

Patch Set 3:

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Message from Phuong Le (Aiden):

Patch Set 3:

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Message from Cuong Manh Le:

Patch Set 4:

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Message from Phuong Le (Aiden):

Patch Set 6:

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Message from Cuong Manh Le:

Patch Set 6:

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Patch Set 6:

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Patch Set 6:

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Patch Set 6:

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Patch Set 6:

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Patch Set 6:

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Patch Set 6:

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Message from Phuong Le (Aiden):

Patch Set 7:

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