Start block at loop var free

This ensures that code directly before the loop var free is
separated out (and will generally be eliminated as unreachable).
This fixes some assumptions

Start block at loop var free

This ensures that code directly before the loop var free is
separated out (and will generally be eliminated as unreachable).
This fixes some assumptions we have that unreachable loop var free
blocks start with the loop var free.

Fixes oss-fuzz #39395.

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