bugfix: make sure to split predicates before planning them

[systay]

Description

Backport more fixes to the 18 branch

Related Issue(s)

Fixes #16990

Checklist

• "Backport to:" labels have been added if this change should be back-ported to release branches
• If this change is to be back-ported to previous releases, a justification is included in the PR description
• Tests were added or are not required
• Did the new or modified tests pass consistently locally and on CI?
• Documentation was added or is not required

[vitess-bot]

Review Checklist

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[arthurschreiber]

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