Fix check for unreachable blocks in merge-return (#2762)

Merge return expects unreachable merge block to look a certain way, and
unreachable continue blocks to look a certain way.  What if an
unreachable block is both a merge and a continue?  The continue is
suppose to take precedent, but merge-return implements it with the merge
taking precedent.  This change flips that around.

Fixes #2746