[LTL] Fix verif_* -- these are statements not expressions, no ret value.

[dtzSiFive]

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• Bugfix

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Release Notes

Fix LTL verif statements.

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[jackkoenig]

Unrelated to this change, we should maybe make them expressions in the same sense chisel3.assert is so that labels can operate with Chisel naming behavior, same convo as #4001 (comment).