cmd/compile/internal/syntax: simplify jumpsOverVarDecl

[callthingsoff]

To make it a bit clearer to read.

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Message from Gopher Robot:

Patch Set 1:

(1 comment)

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Message from Jes Cok:

Patch Set 1: Commit-Queue+1

(1 comment)

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Message from Go LUCI:

Patch Set 1:

Dry run: CV is trying the patch.

Bot data: {"action":"start","triggered_at":"2025-08-15T13:01:20Z","revision":"9f233fe541b231ef04a17df5fc400e99e7780210"}

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Message from Jes Cok:

Patch Set 1: -Commit-Queue

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Message from Go LUCI:

Patch Set 1:

This CL has passed the run

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Message from Go LUCI:

Patch Set 1: LUCI-TryBot-Result+1

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