unreachable was true if the last block is unreachable in the linear cfg,
but it should also be true if it is unreachable in the logical cfg.

Fixes dEQP-VK.graphicsfuzz.for-with-ifs-and-return

Signed-off-by: Rhys Perry <pendingchaos02@gmail.com>
Fixes: 8d8c864b


    ('aco: improve check for unreachable loop continue blocks')

Reviewed-by: Daniel Schürmann <daniel@schuermann.dev>
Part-of: <!4764>