Fix block marking for two arm math

This would end up taking the successors_count=2 case, even though
we need to treat SWITCH and MATCH differently. This incorrectly
marked a block as

Fix block marking for two arm math

This would end up taking the successors_count=2 case, even though
we need to treat SWITCH and MATCH differently. This incorrectly
marked a block as FOLLOW, resulting in incorrect block pass
optimization.

Fixes oss-fuzz #39380.

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